I haven't modified my Zero yet (Have it since 3 weeks, love it), but I plan to do it with Wii U buttons and dpad. However as Mar8 says it's probably the most difficult way...
I think that the easiest alternative besides 3D printed parts would be to trim a Gameboy DMG dpad. (Or stiffening the back of the original one), or maybe buff/polish the controls (but it would require a particular equipment)
However, in the meantime, I've achieved very great and durable results on the buttons without disassembling the console
By using a fine strip of cloth that I soaked (Slightly) in WD40 , holding it firmly between thumbs and forefingers then rubbed back and forth forcing on the edge of the buttons. Then, clean off the excess with a dry cloth and wait a bit.
Also good for the dpad (WD40 cleans well but is not very durable as a lubricant), depending of the Zero.
EDIT : You can do this with rubbing alcohol too.
In fact these controls have a too snug fit, so any even really tiny extra-thickness left by the dirt or else will jam and "squeal" ^^ (and in case of dirt already present, silicon grease maybe wouldn't help)
I hope it will turn out great, good luck