If you are set on using plexi that's fine, but don't be affraid to try glass.
You can cut glass the same by scoring with a $2.00 glass cutter and
some WD-40 as a cutting oil, place a dowel under the score line and
press off to the sides on a flat surface, snap, clean cut. You can use
sand paper (I use my belt sander with a wornout belt) to remove the
sharp edges so you can't get cut. Then run the silicone along the edge
(small 1/8" bead), gently press together, some masking tape to hold the
pieces in place until the next day when the silicone is dry enough. I
used to get the glass cheaper than plexi and it doesn't scratch as
easily.
Yes, I worked for awhile at a glass shop years ago. Replacing auto
glass, house windows, custom fish tanks for customers and myself, table
tops, mirrors, green houses, etc...
But hey, it's all good... have fun.