I had a Tree Lounge and bought a Summit after a few years of use. The Seat-of-the-Pants harness that came with the Summit fits nicely on the Tree Lounge. You just need a carabiner or some sort of ring to attach it. I gave the SOP harness away with the lounge when I sold it in favor of a vest style harness that would also work with the Tree Lounge.

I bought one of those stainless rings that you complete the loop with by screwing shut. It's got a 2100lb rating and holds the canvas straps nicely. I also coated it with some rubberizer to silence it.

I highly recommend a full vest over the belt style harness the Tree Lounge gives you.