paper-inserts for an exterior
sign? seems chintzy....
seems easy enough to do in-house, however, but I've never seen it as an "off the shelf" item from any supplier.
edited to add: both pics appear to be simple extrusions, cut to size & bolted/screwed/rivot'd on, with a piece of acrylic/polycarb that's thicker on top/bottom than in the center by adding a strip of <1/8" material.
if it were me needing to make this for a client on the fly, maybe design it around the standard cut sizes of Avery peel&stick material that comes off the shelf at local walmart/officesupply. client can print a sticker for their special instances, and slap it on. won't fog-up with humidity, or fill up with debris over time.