Any armouring of a lipo to prevent it getting damaged by a spinner should be down to the builder and not a part of the rules. I believe the 2mm metal base requirement was more to do with preventing a lipo fire from leaving a bubbling mess of plastic on the arena floor, especially in featherweights as a lot of bots use that for their bases.

I've not had 2mm metal in any of my beetles that have been using lipos and, as far as I'm aware, it's not actually in the rules in black and white, so you're perfectly free to run without any metal. If you don't want to make your entire base out of 2mm metal, you can always just put a localised piece under the lipo. I did this on Onyx and Reaver when I switched to lipos as they used polycarbonate and HDPE respectively for their baseplates.