Bags like that have straps like a backpack - instead of looping it over a spare, you could build a frame that attaches to the rear wall/door and affix the trashroo to that. Regarding the size, anything bigger seems hard to manage once full and especially hard to dispose of if you're not headed straight home (eg - in a town between sites).Sorry guys, I think I probably confused the matter.
Looking for something like this, but larger. Buying bags isn’t a problem, just need a place to put them. This if for off grid stuff, at a campground it’s a nonissue.