I haven't tried this with a spring mattress, but I'm pretty stubborn and I'd like to believe I'd get it to work. The thread interested me though because I have, in the past, fit both a king-sized mattress (foam, compressed) and the king-sized bed frame in a 2007/2008 Fit. Of course, this was from Ikea, and there was probably magic involved. But all the windows were closed, the hatch was shut, and I was comfy in my seat. But I did have to buckle the passenger seatbelt because it was chiming from the weight on that seat. (Also, initially I tried to fit all this in my friend's Tahoe, but we couldn't get the back doors shut so we had to bungee them, and it was January and it was freezing!)