You could do a "do-it-yourself" job, if you're up to it!
You can buy a small animal playpen, that looks like short fencing. You can actually use the panels from the playpen on the outside of your wire cage to make the gaps smaller. You can attach the playpen panels with zip ties. Even if you needed to buy 2 or 3 playpens, it still might be cheaper than custom plexiglass, plus the panels still allow for good air circulation. Just an idea!
Best wishes! Degus are such smart and stinkerish little wiggle worms. lol