Cut a thin piece of clear acrylic to cover the entire triangular area. It'll be raised so get the exterior outline right. Could even get someone like Ponoko.com to make it but will cost a bit. (Alternative...3d print inverted with a clear filament. Want the cleanest (build plate) face for the top.
Black paint top and sides after masking with a vinyl cut logo you can peel afterward. Or black paint then laser etch the logo. Suggest multi paint coats for full hide. Low wattage 1-10w blue diode lasers are plenty to "cut" black paint.
Sand the backside with fine sandpaper 400grit or so to make it a diffuser.
Tack glue it on with small dots of superglue well away from the actual light source area (after just holding in place to test and see if it lights up the way you hope).