How about siliconing pieces of glass to the back and bottom of your tank to make your island? You'd need only 3: two sides and the front. People do this all the time to make sumps, etc.
Framing shops can cut glass to order if you don't want to cut it yourself.
Or if you want to use acrylic...it's pretty easy to cut. I buy it from Amazon. It comes in various sizes and thicknesses, and you get it cut to order as well.