Tank Mates for Betttas

My tank is 16" long, about 8 1/2" front to back, and 10" high. The lfs told me it was a 5G. I originally bought it as a hospital tank but I got another 20G for that in case I need to put bigger fish in quarantine. I also have a 10" tall cave like ornament, it would be as tall as the tank. Is it safe to put this in there(after I sterilize it of course), or should I get something else? Thanks again for all your help. This is a picture of the ornament in question.

Again as a tank topper, you could also cut the plastic canvas to fit directly over the enclosures surface. Junst an idea.

Plastic canvas is an idea..... Wouldn't I have to cut holes in it? I would have to cut at least one hole for the heater. It hangs on the back of the tank. It is a small heater, it keeps the tank at a steady 78 degrees. I currently have a bubble filter in the tank as well. Should I take that out? Thanks again all, I am going to go buy the betta soon. I will probably go look tomorrow or Wednesday. I must say I am so excited about it too. I love Bettas, they are my favourite fish by far. I'll post pics when I get him.

the canvas you can get in the craft section of walmart. it is easy to cut and will let air in/out. it is like 24 cents a sheet!