They're made to fit a standard pack, and should accomodate standard LiPo packs, too.

Your JRX-S may have been modified if it can't accept standard battery packs.I have pics where the battery shown is a Venom 2S hardcase LiPo installed (the same packs I use), so it will take 2S, but without pics, it's hard to tell you what you need to know.
I don't know what could be holding the packs from being installed, so pics would be beneficial.