Hubs built a sleeping platform for our Toyota Sienna using angle iron for the legs and crosspieces and thin plywood for the top. Lots of storage underneath. Lighter than building with 2 x 4s.
I've had some great nights sleep in the van.
For screen on the open windows to keep out bugs, we get fiberglass screen mesh by the roll at a big box home store, cut it a few inches larger than the window all the way around and hold it on with round magnets. The fiberglass screen mesh is easily cut with scissors. The magnets are about nickle size or a big larger. Cheap and easy.