To keep the heat from convecting back out of the panel, there must be some kind of transparent cover for it. Usually you'd use glas or special acrylic glas, but that would be too expensive for such a project. So I decided to cut out the straight segment of PET water bottles and iron it. To really get the segment as flat as in the picture, you need to put the segment between two cotton cloth and carefully iron it on the highest level with steam and pressure.

For the project you need about 25-30 pieces.

After you have enough to cover the panel, you can glue the pieces together with silicone, or weld them together with a soldering iron. For the latter method you need to put a piece of baking paper between the pieces and the soldering iron.

After finishing that, you should have a nice big plastic cover for the panel, which you can fix to the panel frame with some nails and silicone glue.