I’ve made similar coasters a while ago, just with a stone tile as the base. Despite having to endure a cup of hot tea every day, it’s holding up very well so far and stays clean.
The PETG did minimally deform after months of using it, which is both good and bad. On one hand it’s now formed in a way that perfectly fits the specific cup I use, but that also means that it’s become a bad fit for every other cup or glass.
Your comment about using TPU has given me the idea however to make the tile reversible with the current PETG on one side and Flex material on the other.
That’s a bummer. I’ve been using the forked version as well, and even that dev has been annoyed with Google Play enough that it’s only released on F-Droid nowadays.
Personally, I don’t think it’s an issue only releasing only on F-Droid, because the people interested in Syncthing wouldn’t be deterred by that if they’re not already using it, but I totally get why that might sap the last bit of motivation the dev has.