Grab a Lipo? Plenty of those that'll fit inside a KAC RAS replica without breaking a sweat.
No jury rigging required, no fake PEQ box or anything else like that. If you've got to make do with what you have that's absolutely fine don't get me wrong, but NiCd & NiMh are the battery equivalent of a VHS tape.
My Army L85A1 with "noob tube" and maglite flashlight and red/greendot. (photo 2010) and in 08-2013 i had this:
ICS L86 LSW including laser and A&K elec. drumbox
Army L85A1 (now with SUSCAT scope. standaard version)
and here with "noob tube"
TM M1911 SF