The wire fencing that forms my frame is probably stronger than what you have because all of my chicks, even when they were older, could wrestle around up there on it and I had no issues. I think early on someone made a frame from just hardware cloth and that wasn't very successful because it was too flexible, the same thing that it sounds like you are experiencing.
Back a page (or maybe two by now) you will find @aart posted a shot of her revised frame. She uses an oven cooling rack, so rather than having a cave she has more like an EcoGlow. You could try something like that if you don't have access to any stiffer fencing. I know @aart is very happy with her setup. Me, I like my wire welded frame because I prefer the arch - the cave.
@mobius one of the beautiful things about this system is that there really is no need to rely on any kind of thermometer. Early on in the thread @azygous asked me what the temps were running under there. I had no idea, but curiosity got me and I put a wireless one under it just to see. By then the chicks had already been enjoying it for days. The room was 69 degrees. Under the cave it was 82.5 and they were just fine! They regulate their own heat with this system, so fussing over a degree or two just isn't necessary. The heat isn't spread out over the entire brooder, it's just at the chicks' backs, right where they need it.