Back in the day a Veco tank would fit between the spars.  I used the 4 ounce as my Fox's were hard to start hot.   There were to tubes,   One exits the bottom clearing the leading edge for the fuel pick up and the top one barely in the tank for overflow and pressure line.  Don't make a hole through the leading edge.   This was the set up shown on the Goldberg plans.  The thing about this set up was the pressure line could be pinched off while starting.

Floppy balloon tanks work just fine! Big balloons can hold over 2 ounces of fuel, just right for casual sport flying. Do a search on balloon tanks for the how-to.