The resins that you get in a home center or automotive store (Bondo et all) are not high quality laminating resins. They're adequate for some small repair jobs but that's about it. I have used this in a pinch before to try and build some small molds but had very bad results (lots of deformation and twisting). I have had great and very stable results with the "404 tooling resin" found here: http://www.shopmaninc.com/polyesters.html. At $32 a gallon, it's comparable (or even cheaper) than what you can buy locally.

As far as the release wax goes, I am not inclined to think that a liquid wax will do the job adequately. I'm really not affiliated with these guys in any way, just a happy consumer...but here's a great list of release waxes: I use the Partall film and paste wax with great results. The film is an additional time consuming step (and an opportunity for dust to contaminate the part surface) but I have not gained the confidence yet in the release waxes and am nervous about destroying my tooling. http://www.shopmaninc.com/moldrelease.html

As far as filling the opening, several options. Clear packaging tape will work just fine if you are careful in the application, but the edge of the tape will show on the outside or have to be cleaned off. Another option is to fill the wheel pant with plaster or foam, shape the opening and fiberglass or bondo. Personally, I would probably go the plaster route and use bondo and finish off the plug opening with some paint.

Second thought, you could also fill the opening with modeling clay...won't necessarily be a "perfect" surface but should be good enough for sealing it up.