I'd say forget aluminium all together. One hit from a spinner and it will get buckled and need replacing. Polycarbonate is decent stuff, but anything less than 1mm isn't really up to the job. You can buy and A4 sheet of 1mm polycarb from ebay for less than £2, and that will be enough for 2 or 3 antweights. Also, as others have mentioned, HDPE is very good at absorbing impacts and is also quite cheap. It's not too good against cutting weapons though, Ripto easily cut through 2mm HPDE on one of my ants at the last AWS.