Any place that sells acrylic sheeting should have or be able to source PVC sheet as well. 4-6mm should work.One of the other pictures actually uses a white body - it helps to reflect the light internally.
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Where would you get the PVC to use for the top and base?
The reactor is still empty and I have 15m of the strip lighting from the camper I am about to replace if you wanted to give it a go @RobWhy not a TLF550 reactor and wrap the LED's strips around the outside. Seems like too much trouble with an internal light tube when you could get more LED's wrapped around the outside. Especially if you are thinking of using std LED's. I think you can probably get a 1m strip of LED's quite cheap.
I know @macca_75 has a TLF550, and I have a 2m strip of lights to wrap around it so he can give it a try.. only problem with this it obviously has to be external