I've never done this before - so I'm making it up as I go along. I really would not recommend doing this unless you are well experienced in this kind of activity.
First some caveats: vacuuming forming involves melting plastic with a lot of heat, so some precautions are advisable:
Lighting diffuser panels are made for suspended ceilings, and are very cheap since they are mass-produced. There are several different patterns - I used the "cracked ice" pattern as it looks semi-random. The panels are available in either residential grade styrene, or commercial grade acrylic. The acrylic are more expensive, but seem to vacuum-form only about as well as the styrene ones.
I cut these panels in half as they come in a 2'x4' sheet, which is exactly right for two roundels.
On the first try I oiled the mould with cooking oil to stop the panels from sticking, but it turned out that they don't soften enough to get sticky, so this was probably not necessary.
The heat source here is a flame weeder, which puts out a lot of BTUs - I'm not sure how many, but more than my camp stove, and more than a barbecue I think. The wood gets a little scorched, but doesn't smoke or burn.