Well, my little ribbing scheme didn't work the way I thought it would.
I was originally thinking I could build up the ribs using more Plasti-Paste, first by laying down strips of weatherstripping with a 1/8" gap in between, then troweling the paste over the whole thing and working it into the 1/8" slots thus making up the ribs. But when I couldn't find weatherstripping of a suitable size, I turned to foam strips as a possible solution. I cut a bunch of those and sprayed one side with adhesive. But that didn't work either because the strips wouldn't take to the curves very well and the ends would pop up as a result.
So I am now in the process of laying the ribs down one by one, using 1/8" thick styrene strips cut to 1/4".
Sorry I don't have pics just yet, my DSL is down at home. I will post them tonight if I can get my internet back up.