1999 Ram 1500 Extended Cab Aftermarket Rear Seats

Im driving a 1999 Ram 1500 EC and I'm trying to figure out a way to mount a 29x19x12in sub box. I want it completely secured so I'm thinking my best option would be to remove the back folding bench seat and mount it to one of the side brackets, but I'll have room for at least one seat and was wondering if there's any available aftermarket single folding seats. If not I'll probably end up cutting the one I have to fit and modifying it. I've tried searching for parts online, but I can't find anything that really talks about it. Any advice will help.