Plexi is simple to cut... just score your line with a carpet knife a couple of times... then snap it... you need really thin Plexi for something like that.
how big is the tank? they sell tank dividers w/ plastic and screen for commonly size tank. otherwise, just go and buy a piece of acrylic and drill holes through them.
My 40 breeder is 36 inches long by 18 inches deep by 17 inches high. I bought some plexiglass(.08 thickness) and had a hard time cutting straight. I bought a plexiglass cutter but it does a bad job. LFS sells divider but way over priced.
This is a totally different idea but you can always do what I did. I built a a pressurized divider.
Zip tied a plastic nylon mesh creen to to 3/4" PVC with holes drilled in the top portion like a spraybar. Hooked that up to a powerhead for circulation over the screen. With all air/water flow algea stick to this thing and stays off the acrylic. Might help be a food source for you pleco fry.
I've not scraped algea from the front yet, not even once...