Eric, you make it sound down right _simple_
I've been accused of that before.
It can be. Even drilling a bulkhead fitting into the tank is strait forward. It helps that I've done it a few times and spent 40 years working as a machinist/fabricator. I drilled the last bulkhead fitting hole in a new motorcycle tank in a friends driveway with borrowed tools. I rode that bike, with a new fuel cell mounted, home the next day. OR to UT that time. No issues.
what complicates fuel cell integration on a motorcycle is the other things we want to do. Carrying passengers throws a big wrench into the simplicity of mounting a fuel cell. Your definition of quick removal may vary. A bike with a one piece seat creates issues too. Having your load platform over the pillion area (and) the luggage rack area, keeps weight forward. Covid has screwed up some of the good sources for custom fuel tanks too.
Find tank in size you want. Contact seller and attempt to verify that said tank really is going to be that volume, and will come with a letter stating it's exact volume, with that volume stamped on the tank. Preferably where it can be read when it's mounted to the bike.
Decide if you're going to use a bulkhead fitting or go with the vent line method, spend some time reading and understanding the two methods and limitations and benefits of each.
make sure you have a non-vented cap for the fuel cell you're ordering. Make sure you have a fitting in the top of the tank that can be used for a roll over/vent valve with a hose fitting on it so you can fun the vent line down and away from the bike.
figure out your fuel line and vent line routing, pick a spot for your fuel filter, preferably where you can see it easily or w/o doing more than lifting the seat. Now find fuel filters that will fit in that spot, buy 2 or 3. Avoid glass and screw together filters. Clear plastic is best so you can visually see contaminants. Decide where you will run your fuel lines to the main tank and where you will install dry break fittings. Remember, you may need to lift the tank for service, so dry break fittings need to be able to move enough to do that w/o removing them.
Buy some dry break fittings. I like outboard marine ones in the Chrysler Force style. Usually nickel plated brass and durable w/o being too expensive. One end needs a NPT to hose barb fitting added. The other end comes with a hose barb. I like 5/16" I.D. fuel lines. Larger are more difficult to route, smaller flow much slower. On gravity feed systems this can be an issue. On vent line ones, not as much.
Find a valve that is good quality and has a big enough lever that you can blindly reach back and turn it. Figure out how you're going to mount it, if it's not screwed directly into the tank, (my preference).
Some aftermarket tanks come with mounting tabs welded into the tank. So you just need to figure out how to make a flat plate bolt to the bike in the space you have. Level? Or forward sloping? Is the fill hole/neck going to be an issue? Can you alter that by installing a different neck?
Once you have a plate solidly mounted to the bike, larger enough for the cell, make sure to use a vibration isolator between the tank and the plate. Some cork, some rubber, something as simple as equipment feet that already have holes through them or a piece you cut from an old mudflap to the size you need and drilled a hole through. This will keep the mounting tabs from cracking at the welds. Or at least slow that process down.
So to summarize:
Find: tank,
Find/verify non-vented cap
Find fuel filters, bulkhead fitting, (if needed), dry break fittings, fuel line, vent line, roll over valve/vent fitting (hydraulic hose shops, race shops, marine stores, auto parts stores)
remove pillion seat, locate bolts/bolt holes, use cardboard to create a shape that will fit and mark holes, decide on bolts and spacers needed to mount plate to bike. Mock up with cardboard/foam board.
Go have a local water jet or fab shop turn your cardboard/foam board template into a piece of aluminum or steel. I prefer 3/16" thick aluminum, but sometimes it's about what's available at the right price. Pick up template and finished mounting plate. Deburr as needed, use the template to drill the bolt holes, if they didn't do that for you. Farm stores are a great source for spacers. NAPA stores usually carry dry break fittings for marine applications.
Mount plate to bike. Mount fuel cell to plate, making sure you have clearance for the valve or fittings. Put seat back on and check. Looks ok, then start running hose. If you're going to drill the main tank for a bulkhead fitting, use a step drill, remove the tank from the bike, remove the fuel pump assembly, drain the tank, tape the opening with painter's tape to protect your hands. Use a spray can cup and tape it to the inside of the tank where you're going to drill to catch most of the chips. (duct tape there, not painter's tape). Mark your hole on the outside, put the tank back on and double check that you'll have clearance for the bulkhead fitting. Trim down the excess fitting on the inside end of the bulkhead fitting, leaving enough for the nut and Stat-O-Seal washer. Locate a wrench to fit the nut before you put the nut on inside the tank. (shorty combo wrenches work well)
Install.
See, simple.
If you have a bike with a charcoal canister, it's sometimes even easier to remove the charcoal system and use the fitting or holes for it's return to the fuel tank for your aux fuel line. This works well for aux tanks that sit higher than the main tank. Like F650GS2s that had the main tank under the seat.