I've been using a 4 gallon cell like this:
https://www.summitracing.com/parts/rci-2040a/overview/
Advertised @ 4 gal, actual is closer to 3.5 using the rear fittings. I cut those off and had them welded up, then added a bung on the bottom and get 3.7 gal, which gives me 10.3 total. I got mine used much cheaper, altho it was advertised as not having fuel in it. It's worked out great for me for some 100K miles now. I do use a pump and like that system, butt I know many prefer not to run them that way.
Not as spiffy as some of the others mentioned here, but a good system regardless. Even with the mods in welding I had to pay for, I was still into mine for $150 or less. My range is over 400 miles total, but I get fuel usually in the 350-375 range depending on speed avg and fuel quality.
I've been going to make a Tanji/Maple type flatbed tank and take the capacity out to 4.7 g, with maybe a piggy back tank for another couple of gallons as I don't really rally, so the limit isn't a big deal, but I know value down the road comes back to IBA limits. My idea is different than anything I've seen, but I think it would work great. There are many options for aux tanks on FJRs and some are very cost friendly, but anything special built raises the price considerably. You just have to decide what's important to you for LD capacity, whether you carry a passenger, etc.
The modified main tanks work great too, but for me is a no go. I have a big tank bag and basically live out of it on the road, so the added height of a modified main "Frankentank" would make that area too tall for my use.