10mm HDPE would actually be about 250 grams heavier than the 3mm alu. I have this problem in my machine, too, as in, the baseplate eating up a huge portion of the weight limit. It depends on how structural your base is, but I have been able to drop mine to just 1.2mm ti (only has to hold the electronics in). I wouldn't trust that to handle all of the forces of the ram on its own, unless it's supported around the base of the ram well, but point is, it'd save ~500g.

Saying that I'm not sure how much more I'd trust soft 3mm alu to support those forces, so it's probably a much for a muchness. If there is some sort of strut (some lengths of the 3mm alu on-edge perhaps) support under the ram then I'd expect it to be fine.