AEG Battery Butt Compartment aka APS Patriot butt tube
reference picture from http://img01.militaryblog.jp/
making a butt battery compartment from a refillable ketchup cylinder container
sample picture of a ketchup refillable container
look for a plastic butt adaptor, same as the pics and attached the cover of the ketchup container and your done!
finished m4 butt battery compartment
or you can also use a pvc cover & an empty silicone sealant container...enjoy your battery compartment