Sullivan Flrx Tanks
Anything Sullivan makes is great and they are a top notch company in my book. The flex tanks are great and they say you can reform them somewhat to fit into tight spaces with a heat gun and I bought two for a twin project and the flexibility enabled me to literally squeeze them in. Take some reenforced shipping tape and make a handle on the rear of the tank and you'll be able to get it out. Personally I like hatches for access to everything.