UHMW, or most woods sealed with preservative should be good. I can't imagine UHMW soaking up anything present around machine tools. There are too many rubber-like materials that react to coolants.
I'm thinking a shallow Vee cut to sit less than flush AND not touching bottom would be a good combo.
This works for milling machines too, of course. There are commercial 'seals' made of aluminum with drain holes along the length. I suppose, personally not thought much about tee-slots until present position...most work gets flood coolant on the lathe, and spray-mist on the mill. They have aluminum fillers with drain holes. Top is about 1/16" diameter, no countersink, but back-drilled larger, 1/8" mol. The lathe still needs one or another type of deflectors for the chuck.