Shower corner filler strips
Filler strip for KOHLER® Tailored vanities. Designed to work with the vanities in the KOHLER Tailored® vanity collection, this filler strip is used when the vanity is placed next to a wall, to provide extra clearance and allow the vanity door to open freely. For use with Jacquard.
Product Availability by Store Location
You might try a flexible plastic "caulking strip": enter image description here. I have used this several times and will share what I think are keys.
Filler Strip for Vanity | K | KOHLER
Description:The degree back fits into corners, revealing a smooth front that rejects water, and is pleasing to the touch. Thoroughly dry the entire cleaned area and waterproof the gap between pan and surround before covering it with any trim. It cuts just like wood. It has a degree back with a round front. If you have a shower that's built with any type of waterproof panel instead of the typical one-piece fiberglass enclosure, PVC trim can benefit by sealing everything tight. That is, so the door would hinge near the centre of the cabinet, and the hardware would be near the edge of the cabinet? It has a reverse or concave curve, but is not as easy to work with because of mitering required on corners for a good fit.