Excellent idea, Chris, especially since you already had some Plexiglass on hand! I also am a total believer in the P-touch labels and have been using them on slats from (used) mini blinds that I've hoarded when people were getting rid of them. So far mine are on their 2nd year in my garden (although not out during the winter since I use them in my veg garden) and look as good as the day i made them. I think the first thing to go may be the plastic from either the blinds or the Plexiglass, from the UV rays - but the labels themselves will probably still be legible to make new ones. I've been truly amazed at how well they stick to the plastic slats.
