Check out the wheelbase of the Jet9 - I don't think you could have fit anything bigger, lengthwise.
www.ninerbikes.com/jet9As I mentioned. I fit a MUCH longer bike in, but with more disassembly.
A Rigid 29er L would probably be okay. You might have to flip the fork around and remove the brake caliper, this shortens up the length a lot. After that there isn't a whole lot you can do to shorten/lower a rigid bike.
If you are worried, measure your bikes length without the fronjt wheel and I will measure the box.