In Storage or on Display?
Post-Script: Sure, I'll admit that the resulting typewriter cover is a bit crude-looking. But it's entirely functional and, with having made cardboard templates, I can make others if needed in the future. This also raises the possibility of displaying other typewriters in my collection in a similar manner around the house, using these plastic display covers. I would just need to fashion custom templates for each model of machine.
Here are a few descriptive images on how I made this typewriter cover.
I measured the machine's external dimensions, then calculated the size of all the panels required for the cover.
I then laid out these measurements on thin cardboard and cut & taped them into templates. The template for the top panel I only made one half of, in order to save on cardboard. When laying out the template onto plastic film, using a Sharpie marker, I mark one half, then flip it around and mark the other half.
I've had this low-wattage solder iron for decades. It has a rounded, bevel tip that's perfect for this application. If using a higher-wattage iron, a light dimmer switch can be rigged to reduce the wattage of the iron. Or use more layers of thicker paper. The thickness of the top sheet of paper, the pressure applied with the iron and the speed at which you move the iron all determine the melting/fusing of the plastic film, which also should be varied with the thickness of the plastic itself. You want to fuse the plastic so it doesn't peel apart on its own but without damaging it through overheating. When needing entirely straight seams, a metal straight-edge can be used as a guide for the soldering iron's tip, while this method also permits freehand curved seams to be made, as might be required for the curved gores of balloons.
Close-up detail of a corner seam. I trim the excess plastic film from each seam, to make it look a bit neater.
Typecast via Corona 4, images via Fujifilm X10.