Your assumptions about the fuel tank are correct. It is quite acceptable to retain the tank with a bit of balsa stock. If there is no padding on the bulkhead cut-outs for the tank, try to fit thin foam padding between the tank & the cutouts -- this will help prevent fuel foaming (some Nexstars that I have seen have moulded in padding).

Thin your (30-min) epoxy with anhydrous iso-propyl alcohol (get it from a drug store). Mix the epoxy, as per directions, & add just enough alcohol to make it easy to spread with a brush.