I must admit I am confused. What you refer to as a tonneau cover is really just a cover for the top. My roadster came with that with the exception of the two bumps in your picture.

A real tonneau covers the cockpit area so you do not have to put the top up when you park or lets you just cover everything except the drivers area while you are driving. They are normally made out of vinyl and snap in place. You could get one fabricated by a upolstery shop. I'll try to get a picture if you want.