I had been using 3 pieces of 18650 in parallel to provide power to my Nexstar mount via the external power socket. The internal AA battery compartment is also used as a backup source.
Last night I tried to pull out the AA battery compartment and I found it easy, it was just fixed there using double side sticky tape. I tried to fit in my 3 pieces 18650 battery pack there and it has more space than enough, and such a modification is easily reversible too!
It even has enough space for a electrical coupler and thus no soldering required. I'd love to find/make a replacement battery cover after the modifications.