Instead of masking with tape, I made a 3D printable bezel that fits in the aluminum bezel of the BoxyPixel Aluminum GBC shell (https://www.boxypixel.com/collections/frontpage/products/game-boy-color-ags-101-machined-housings) and covers up the blank area of the screen. See pictures for after and before (in that order). It should be a perfect fit so depending on the tolerances of your 3D printer, you may need sand or trim slightly. This is directional and only fits (properly) one way as all sides are different sizes. The top is 2.5 mm, the right is 2.15 mm, the left is 2.0 mm, and the bottom is 2.2 mm. The whole spacer is 1.2 mm tall (z).
Please measure your device before printing and installing as there have been revisions to the shell since I purchased mine and dimensions may have changed. Additionally, the LCD may have a smaller gap if you have no LED and if you did not secure the LCD with tape or adhesive. This is not required at all for this build. I recommend printing in black material but really that's on you.
And this should go without saying, but don't put any sharp objects near your LCD. it is very easy to scratch the polarizer and extremely difficult to repair properly. So if this bezel gets stuck in there, bend it from middle with your finger and print a new one instead of sticking a knife in there to pry it out to save it and trim slightly. Learn from my mistakes.