I recently had to pull out my bathroom sink and cut a hole in the wall around the pipes to do a repair. The person who owned the house before my had used galvanized pipe nipples coming out of the wall with brass flanged elbows. So I had a classic case of galvanic corrosion on my hands.
After doing the repair I decided to not get into doing any drywall work to seal up the hole and instead used a 1/8 piece of luan ply to basically panel over the small area under the sink. Had gaps in the panel where I cut the pipe holes. To get a tight seal around the pipes I decided to laser cut some flanges (escutcheons) from 1/8 ply. The drain pipe cover was just a round flange, but I decided to get a little fancy with the water supply covers. Not that anyone is going to see them but I was like what the heck, I got a laser so why not. Did scallop shells which are cut in half and attached with screws so I can remove them if I ever need to.
(Edited to change to correct terminology of the device courtesy of @Domm434