Celeryman done the job right, weld in new pieces and ferget the glass. After opening the rusted holes for welding, bang it with a hammer to loosen as much rust as possible and shake out. After welding it up, drill a 1/4 inch hole near the top on the front side and pour in some undercoaring material like what is sold by Eastwood. Roll the support around to allow the inside to be coated. This will protect it from future rusting. I did my entire frame with a similar material.
Glassing over a fiberglass hole can be done easily. The resin will not stick to plastic like a drycleaner bag and the like. After tapering the edges of the hole to be filled, make a form to back up the hole and sandwich the plastic between the form and back of panel. Tear a few pieces of cloth, not mat, to layer into and over the hole. Never use mat near the surface as the weave will show through the paint. After everything is set-up, it is time to mix the resin. I like to wet the tapered edges with a little mixed resin using a 1 inch paint brush. Lay the prepared fiberglass cloth on a plastic sheet and coat it with resin. Layer a few pieces as needed and coat as you layer. When done with that, turn the plastic sheet over as you glass it into position over the hole. Too much resin is not good, so monitor the amount used. It is important that you remove all the air bubbles before it sets up. This can be done by dabbing with the tip of the brush used to coat with resin or rolled with a fiberglass roller. Acetone will clean the resin from the brush, roller and hands. Did I say wearing latex gloves is a good idea. This method has worked for me, but may need to be modified to suit your need. In some cases you may need to use sheetmetal screws to hold a form in place and body pieces together. After it sets up, grind it smooth and fill low spots with bondo, prime and paint.
