As none of you mention it, i'm guessing my original plan of angle grinding the HDPE down to size isn't advisable or will otherwise produce not too good of a cut?
Jigsaws aren't too expensive, although I need to buy (or just build) a work bench still before construction fully gets underway, motors & battery arrived today, so literally only thing left to do is figure out how to work the HDPE I bought (2000mm/500mm/10mm) in the best manner. Will need to buy some thicker stuff for key areas i know.