You might want to consider staggering the seats, so the passenger is a little fwd. Then put the FT to drivers left.
Make the drivers seat a few inches taller than the passenger, so you can see what you are about to crash into.
As far as making your own tank - been there, was a big waste of time & material.
When I changed to the Yamaha engine, I was is same boat as you (ha). Their FT wouldn't fit beside engine (was huge 16 gal). So after $100 of alum & several weeks of chasing pin holes, I had a home-made tank.
Later, when I got my boat legally registered as home-made, I wanted it to be in line w/ USCG regs, in which there is no way you can have a home-made FT.
So I dug out the Polaris tank & made an adapter for the Yamaha fuel line/pump . Polaris tank has certification stamp on it.
Polaris tank is only 12 inches wide - you sure it won't fit on left of engine? It can be low in hull, probably better anyway.
Keep in mind that the 2S jet ski engine is pretty light & is inboard, so the boat won't be a tail heavy as you would think, at least compared to an outboard.
I found some pictures from old thread:
