My only suggestion is not to use plexiglass for your armor. I've never heard of anyone using that, in fact. Try searching these forums for SINTRA, which should point you in the right direction... and with a wealth of helpful information.
YEah, I hate to sound negative, but plexiglass is probably not the best choice for usage as armour. You might be able to do some bending and shaping with it if you use a heat gun, but if you cruise through TDH for a bit, I think you may find some better alternatives for your armour
I've cut out some more peices and I think the best way to get it to not crack is to not go on any even fairly rapid turns while cutting, but to just saw a square around the peice and and trim it to size.