I made the very thing you are asking about. I used chuck gadd's design and scaled it way down for a ten gallon.
Chuck's Aquarium Page
The styro and paint and pretty cheap, it's the epoxy that is a little pricey. You can't spray paint styrofoam, it will melt. Why are you opposed to using silicone? I think it would be hard to make stable without being siliconed in place.