Yeah... I haven't revisited this issue other than the small binder clips which seem to help trap the air on the edges. It works, so its holding up.
its unfortunate to hear that even a 3mm thick aluminum plate sags from the weight. That doesn't bode well for a standalone 5mm PEI sheet... which was my end goal.
Supporting the center is probably the best idea. I wish it wasn't static, meaning no height adjust (I have a TAZ5). Maybe I'll make a mold to produce 8mm silicone blocks or cylinders. The silicone will give a little as I adjust the bed corners. I guess its another addition to the next print bed overhaul.
A forum dedicated to the stock LulzBot TAZ 3D Printer