M_Boats: Porta-Potti and Battery...

I'm trying to find a Porta-Potti and battery for my M-15. The little hold that was designed to hold a the Potti is only 11 1/2" by 15" on my boat and I'm wondering where I can find a potti that will fit that space. I checked out some other M-15s on line and saw the Thetford 235 model being used, however the dimensions for the 235 and 135 models are larger than that hold acording to the manufacturer. Can someone please advise me... I also need a battery to run my depth finder and eventually some lights. I would like to avoid using a big heavy battery. Does anyone have any experience in this area? Thanks, Lou Aliotta