RE: Top-Flite 1/5th F4U Corsair ARF Group Build and modification thread
One method to make your foam tank, take your shape and cut some balsa disks of the correct shape at locations along the length; these will act like "bulkheads" . This will establish the bask shape in 3D. Between those "stations' add Foam fleshing in the entire form. Now you just need to sand away the foam to the balsa or light ply disks and you'll have your shape. Use light weight glass cloth to cover and finish. You can add details if you like, like the fill cap, lettering etc.
Bob Violet, bvm models, sells a "stores" release. Its air operated. Tower sells a foremost bomb release, this is servo actuated. It's simply a spring loaded pin that indexes into some kind of ring you have made on your fuel tank.