You can get larger sheets of just the filter plastic. I can't remember for sure, but I think found it at B&H. I wanted to change my small safelights from orange to red, but didn't want to buy whole new safelights. I bought one sheet of the plastic ($16?) and cut it down (with a Dremel, took awhile but worked). I'm pretty sure the sheet was big enough to cut in half for two that would fit the Thomas (I have one that I'm not using).
I think the bulb for the Thomas ones rules out LEDs, but I'm not totally sure. It's not a regular bulb, though.
And I think you can buy replacement ones for the Thomas anyway.