If you are building your own I'd suggest using PVC (not PVCx) for the bottom it makes heating much easier. Plexy covering flexwatt is a interesting idea but I would be concerned with water spills (I have a bowl flipper he flips dog bowl some how...) and water entering the lower space and shorting the flexwatt. I am sure that careful sealing would prevent this but I am also sure it would need to be renewed every now and again.

Here is mine...

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