Is the roof panel a positive to make a mold from? If it is, you really meed to add 2 or 3 inches in height to it, you will never get glass to lay nicely against the table, It will be some much easier to do if taller.
Also light weight drywall filler is you friend, the stuff made by red devil is like marshmallow topping until it drys. It does not shrink, but dont try to sand it until its dry. It sands and drys very fast, and it a great base for starting you filling. Its will stick to the wood like its going out of style. When you get ready to start putting the smooth paint on it for the final surface finish, just use sandable primer, you can actually polish it to a shine with paper towels.
Then use car wax to coat it, then a final spray of pva mold release from something like a prevail sprayer. I would recommend putting a 1/4 inch hole thru the positive in 2 or 3 places once you get it all smooth, then put tape over the holes. It will allow you use an air gun to blow air in between the mold and the panel and get them to separate. YOU REALLY want to do that. Yes you might have a small spot you need to sand, but it will save you an hour of fighting it apart.
Fiberglass Evercoat PVA Mold Release 8-OZ : Pva Mold Release : Sports & Outdoors
www.amazon.com