I've been doing some research into this, because I'm converting an old bookshelf into a display case. Plexiglass would work, and it's inexpensive. However, it develops a yellow tint over time. Acrylic does the same job, but without the yellow tint. However, clear acrylic sheet is a bit more expensive. I have found some youtube videos on building acrylic display cases. It doesn't look very difficult, and we, as modellers, already have the required skills. Just type "acrylic display case" into youtube. There is a wealth of informative videos.