I've done exactly that and it worked. It's not light exactly light weight and you have to fiddle with it to make sure there aren't gaps, but it will certainly work. And it's CHEAP. Which has some major advantages.
I used a $20 ATHH under a HH Explorer with a cheapo synthetic bag laying in it. I just clipped the ATHH to the suspension of the HH, then used the side tie outs to keep it snugged up against the bottom.
That said, depending on the temps you are probably better off with a pad, just because it's less bulk and easier setup. But that's just my experience.
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