Expanded or Aux fuel tank for 1st gen tundras?

Same problem we always have with 1st gen tundras: Finding someone who makes aftermarket parts to upgrade with.... this time I'm trying to find someone who makes an larger/expeditionary fuel tank to replace my factory fuel cell. I've seen some for 2nd and 3rd gen tundras (Transferflow), but yet again, nothing for 1st gen's. Any suggestions from the crowd??? I've read a lot of discussions about home-shop projects fitting cannibalized tanks that are small enough to fit under the bed where the factory spare tire would normally go and using that as an auxiliary tank, but obviously that involves creating a whole other fuel port and most of those aux tanks feed/pump directly into the main fuel cell, not directly to the engine, plus I'd rather just have one larger cell. I have an incredible amount of spare room under the bed behind the axle since my spare tire and exhaust tips are no longer located back there, so IDEALLY I would much rather just swap out my factory tank for one that expands towards the center and rear of the vehicle (I could easily fit another 50 gallons under there before I'd had to start worrying about weight/balance). If anybody has cannibalized something like this that fits under our trucks, that would be awesome to hear about...
2003 Toyota Tundra SR5, iForce V8
Blue Lake Off Road custom front bumper, custom injectors, custom exhaust from Dirty Deeds Industries, Spec-D Tuning cold air snorkel intake, Optima Yellow Top, Smitty Bilt cable winch, A.R.E. camper shell, homemade steel rear bumper, Rigid Industries 20" spot/flood combo light bar