This message was sent directly to me, forwarding to the PICLIST

The process you speak of I think of called 'Vacum Forming' this involves
having a mould with small holes in it that gets hot. Heat the mold to the
point when the plastic is flexable then evacuate the air form the mould
causing the plastic to take the form of the mould allow to cool, place item
to be packed, then glue on cardboard or use a plastic coated board and heat
seam it on. Should not be too difficult with assorted bits from the junk box.

Cheers Steve.....