AAA Inflatables sells a drop bag with an industrial zipper top. I call it the clam bag. It's not a hard surfaced top, but it's worked well for me. I'm 220+lbs and it can hold my weight for stepping purposes, and has been at the bottom of packing with tons loaded on top. I haven't tested long periods of standing or jumping on it, but that's partly because of the makeshift bay it usually rides in. See video. I don't know what options Sotar frames have for add-ons to help you out on that front.
Hojo - That's a nice cock you have.