To cut plexi glass, you have to keep scoring it over and over before trying to snap it. You can also use a saw. I just am not sure of what kind of blade would be the one to use.
Oh, and it's looking good.
I build something similar in size. Except did not put the pond liner on the inside i put it underneatht the table for leakage, also I used plywood instead of studs. Next step i am putting a hiding rooms in it as well as seperating the sleeping room. I am also putting a small piece of grass sod. Will share those pics as soon as I am done.