The 11.5 gal limit is only for Endurance Rallies. The IBR has that requirement, and many other endurance rallies hold to that as well. If you are only doing certificate rides, there is no limit, so ensuring you stay under the 11.5 Gal limit doesn't matter. Note that the IBA only puts on one event, the Iron Butt Rally. There may be an additional event added, but the other rallies are not put on by the IBA.
On the up side, 5 gallons is a standard race fuel cell size, so there are many options. Securing to the passenger hand grab rails is common practice on GL1800s, if the GL1500 grab handles are equally solid, this would be a simple ratchet strap possibility, making for a fairly easy on and off.
Here is one size, offered in 12"x12"x8"H and 12"x8"x12"H versions that has mounting tabs already on it. This could be bolted to a plate or board shaped to the space on the passenger seat. It would need to be purchased with a non-vented cap, (it comes standard with a vented cap), and a roll over valve/vent can be purchased from the same company to fit it. All the necessary holes are there, except a drain hole. They will probably make one with a drain hole and w/o the pick up tube if asked. If you wanted to use a pump, you could use the pick up tube.
LINK
Summit also offers a variety of 5 gal fuel cells, some with alternative shapes and sumps with low point outlets.
LINK
The spun aluminum tanks are not always accurate on capacity, often being a little over what they claim, but are still another option.
LINK That site has parts as well.
JEGS also has 12x12x8 tanks with and w/o mounting tabs.
LINK
Needless to say, I do not have access to a GL1500 and have no idea what kind of space you have available on the rear seat or if items come off easily that you would be willing to remove when using the fuel cell. Measure the space you have and see what shapes of fuel cells would work best for you, then consider how to secure them. A variety of brackets are available to secure fuel cells in race applications, combining that with a simple board or plate and some padding might do the trick.
You may want to read this thread about
weak fuel pump syndrome.
It looks like with the carb'd GL1500, the simple method to get aux fuel into the main tank is to install a 90º hose barb fitting into the fuel plate that the pump is attached to. Run a 5/16" fuel line from that hose barb to the aux tank drain with a dry break fitting and fuel filter inline to allow removal when not needed. A simple ball valve at the aux tank to control fuel flow. Some good info
HERE. (Ignore the siphon idea and the frightened child w/o experience showing the burning bike pic.
)
Some dry break fuel fittings - JIffy Tite
Male &
Female. Marine fittings have been extensively used by LDRiders as well.
SeaSence Male &
Female. On the male fitting you simply add a hose barb to 1/4 NPT brass fitting to allow it to attach to the fuel line. I've personally used the SeaSense and other Marine dry breaks. You can find them at NAPA and any marine shop, which can be handy. I liked to keep a spare set on the bike, though only needed that once in over a decade of use. The o-ring got damaged when I disconnected to get access to a wiring issue during a rally.
A good product for sealing threads on fuel cell applications -
Loctite 567
If your budget allows for a Fire Creek fuel cell, that's a fine product. You might also contact Ernie Azevedo in Oregon, though it may be too complicated to arrange the bike being present. I don't know if he's made a GL1500 cell before, but he makes fine fuel cells. I've used one of his fuel cells for the IBR and many, many miles before and after with great success. Outstanding person to work with and outstanding quality of work. He may have access to a local GL1500 for all I know.
http://eazcycle.com/