I have always wondered the same thing - how to protect the display window surface when the factory protector will no longer stick.
I have thought of clear tape but with the display a short distance from the window, would it (the display) still be readable? So here goes..
My Wouxon ham walkie does have a display a short distance from the window surface so it should be a good test.
First try with Scotch Magic Transparent tape and it fogs the display - useless.
Next, regular clear tape and it worked fine! I even placed three layers of tape over each other and still no viewing problems (completely invisible protection)!!!
So with three layers (thicker than the original protector), I'm sure there will be enough protection from most minor scratches. Care must be taken to avoid air bubbles when applying the tape though.
Now comes the real test - will the tape stay on in heat or cold? Time will tell with use. And if the tape does eventually peel with age, I can remove the glue residue with alcohol and re-apply new tape without marring the radio's surface. An E-xacto (or razor) knife trims off the areas not wanting to be covered nicely.
Thanks for reminding me of this experiment I've been trying to remember for years! I'm now going to apply tape protection to my other display windows for my radios