That is a beautiful frame. It is even more special since it was from your grandfather's walnut. If you can do that with a table saw, I would love to see what you can do with some other woodworking tools!
I did want to mention that personally I would go from 1/8" glass to thicker glass before I would go plexiglass. I made a swordcase several years ago and there are two thing I would never do again: 1) I will never use lexan (like plexiglass) 2) I will never use oak and try to stain it to look like cherry (my aunt in-laws request).
Lexan is high quality acrylic material with great shock resistance. I still remember one 3' x 4' sheet cost $75 at BORG. The problem was that after I removed the protective film off of it, you could see tiny mini scratches everywhere.