[JulianEdgar]

Originally Posted by aerohead

You were on the right track and got close.
MARINE PLASTICS, of Denton, Texas pre-conditioned sheet ABS, then heated it in an electric pizza oven ( if you will ) then quick-clamped it over a perforated, hollow wood pattern.
When the perimeter air-tight seal was made on the vacuum table, a valve was opened to a spherical vacuum reservoir, communicating vacuum thoughout the pattern, sucking the ABS down into complete contact with the wooden pattern.
Within a minute the plastic took a set, was de-clamped, and sent to a router trim jig for final shaping.
Commercial sign companies form clear acrylic in an identical fashion.
You were very close to success.

I am familiar with vacuum forming plastic.