We tried the Dollar Tree pop-up hamper once and loved how well it fit inside the closet. However, three days later I ended up throwing it out... not because it was damaged in any way, but because DH's dirty socks started stinking up all the clean clothes hanging in the closet!!! (He's going to be mad I just "aired his dirty laundry," lol, but it's true and he knows it!)
I found that the white 13-gallon trash can liners are actually the same width as our roll-aboard suitcases (we only travel with carry-ons). So now, each night we fold our dirty laundry, slide it into a trash bag, twist-tie it shut, and slide it under the bed. We add to the trash bag each night until it's full, squeeze out all the air, tie it shut, pop it into one of the suitcases, and start the next trash bag of dirty laundry. This has worked so well for cruises that we now do this on all our trips!