I wanted a small tray that I could hide underneath the display cover on my Ender 5 (so gently provided by kilroy01 https://www.thingiverse.com/thing:3353394

Since I already had the cover printed and mounted I just glued the rail underneath but I also combined it to a single piece if you don´t already have the cover.

Otherwise just print rail + tray and use super glue to attach the rail to the bottom of the Display cover

The tolerances worked pretty well for my printer, 0.2 offset I believe. If it´s to tight for you, just take some sandpaper on the front and back of the tray part. (Or scrape with a razor)