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:

• This needs to be done outside and away from things which could burn
• protective gear should be worn for eyes, lungs, and possibly hands.
• it's a good idea to have a fire extinguisher on hand (although I haven't needed it... yet)
• Try not to melt the shop vacuum hose (!)

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.

First the lighting diffuser goes in the frame.

The frame is latched closed

Then we turn on the shop vacuum...

It's important to heat carefully and give the vacuum time to do its work - if you overheat an area the plastic will melt and a hole forms. As soon as that happens you lose the suction, and the plastic will begin to lose the mould shape. Fortunately it doesn't stick to the mould.

 

We apply heat... and more heat..

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.

 

We cool it off with a water spray from a garden hose with the vacuum still running so the shape gets fixed before we release the suction.

We remove it from the mould...

 

... And that's all there is to it - all done in less than a minute!

Making Roundels Video (3MB)