Refurbishing mixing box

As part of the ongoing renovation of my enlarger I took a look at the mixing boxes. The polystyrene inserts are very yellow and upon opening then up some of the insert is very discoloured. It's an easy job to replace the polystyrene but is there any reason why manufacturers used this material as it's prone to discolour and even melt. Has anyone found a more durable substitute or is it a case of "if it aint broke"?

Depends how deep the discolouration goes. It my be possible to use an abrasive paper to clean it. There shouldn't be any problem with melting, the heat filter should stop that. I think replacement mixing boxes are available, at least LPL are.

I think they use it because it's cheap and the right colour. Plus when it gets dirty it's cheap enough to replace instead of worrying about cleaning.

If you use anything else I'd worry about colour issues.