I haven’t tried this in a TE, but I did this on other projects. here are several methods that would work.
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[]Encase the PCB in a plastic box, like a project box. Results varies, usually not air/water tight.
[]Hot Glue coating (Semi permanent, but can be peeled off it alot of care is taken before hand and during removal) I would not try to dunk this under water.
[*]Epoxy coating (permanent, you aren’t getting it off without destroying the PCB) I used this method for PCBs that have to get submerged under water.
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Another idea to develop a textured surface for a ball top is use high grit sand paper or steel wool, this will remove any gloss and give the plastic a very matte (flat) look.