Unless you plan on making many copies, you'll spend far more time making a plug, then making a mold. I think you would be much better off just turning or carving the tank from a block of urethane or medium density PRB foam.
I suspect you'll have many problems trying to "mold" the 2-part foams...figuring out the right amount to mix, getting it applied evenly throughout the mold halves, joining the molds before the foam expands too much, etc. Not to mention that these 2 part foams are heavy and stick to just about everything (including mold release spray and wax).
I have tried this - I experimented with some Sig 2-part expanding foam years ago to make wing cores for F3B sailplane wings, all I did was make a mess and ruin a nice set of molds. Just my $.02.