Remember that for total capacity lines and filters are taken into account as well, so it's best not to aim for the exact maximum. I agree that 14L would be a good practical limit to aim for. Sadly, that's an odd size for off the shelf tanks and just under the some what common 15L size.
Just for reference, my first custom Aux tank cost me $1100 USD and was a small production batch by another LD rider. (Dean Tanji flatbed 4.8 Gal unit for the Yamaha FJR1300). My second custom fuel cell would have cost me $800 usd, (another 4.8 Gal flatbed design for a Yamaha Super Ten that I rode in the '13 IBR), but a friend offered to make me one and used that as a basis for creating new designs that he sold to others. Being the prototype bike was both lucky for me, and I'm lucky to have such a good friend. We worked together on the design and fabrication as time allowed over a week or so to get it done. My third custom fuel project was an expanded main tank, taking a 6.2 Gal tank to 8.2 gal using a local to me welding shop that was willing to take on the job. That cost me $600 for the welding work and a total of just over $1000 usd for the finished tank.
So $70 GBP + delivery cost and some frame work is incredibly cheaper, if not as large and as ideal a shape. (perhaps check with some local race shops to see if there are local sources for that tank. Certainly there are other vendors online. You can get different filler necks that bolt to the flat tank, but it would still come out the top in the same place. A taller neck helps avoid spillage during riding and gives you a bit more air gap for expansion. Most of these off the shelf tanks do not have baffles inside, so to meet IBR rules you would have to add fuel foam. In actual function you will not notice any slosh of fuel while riding with that size tank. One of the off the shelf tanks I used was 16L in a plain square form mounted to the pillion area and I never felt it sloshing.
I would still measure your area and draw up a simple idea of the shape and dimensions of a tank you would like. Then hunt through the offerings from the large manufacturers and see if they don't have something close that could possibly be modified. The less change from a stock design will be the easiest for them to do. Then give a few a call and see what they will quote you. Sometimes you get lucky and it's quite reasonable.
You already know the target capacity to aim for. Internal baffles and a side fuel neck, a top vent that will accept a line that can be plumbed down to the rear of the bike and some form of roll over block are also required. Print out the IBR Aux fuel rules from 2019 and read through them, as well as giving a copy to anyone that makes a tank for you, should you end up going down that road.
Basic info
HERE I could not find a active link to the current IBR Aux Fuel tank rules. I did find this copy, which I believe is functionally current for fuel cells. Pages 16-19 are most of what applies to fuel cell use and design.
LINK This article is worth a read though, as it covers much good info and has an excerpt from Tom Austin, who was the final word on fuel cells for many years in regards to the IBR.
LINK
Good luck with your project. I've plumbed in about 5 different bikes, but there are a lot of others here with even more experience if you have specific questions. Don't be shy about asking them. Hopefully someone in the UK will come forth with a specific vendor that has made them a custom fuel cell for the Pan.