If you tried to vacuum form a hot plastic over another plastic item, the plastic ‘former’ would probably deform. Vacuum forming only gives you a limited depth – something like 2:1 depth to width maximum, not the 5:1 or more needed for your part. Can’t think of any process where you could mould this affordably – clear PVC and wire rods probably better
If you vacuum form over something you won’t get a copy, you’ll get something bigger.
You need to take a cast of the inside of it. We used to do this with RC car body shells in school, and it worked. You need to use tape or something to create something that will hold water, then fill it with plaster of Paris. When that dries, you use that as your vacuum form and cut off the pieces you don’t need.
You will probably have to insert some wire into the form to stop it breaking when you handle it.
I didn’t reply to this again as you already have a few options for buying them. Don’t try to mould it, it’s surely a waste of time when you can still buy them.
All modern guards are tinted or made of mesh which is not ideal.
Not so, but if you are just looking at UK shops or using English search terms, I can see why you might think that.
Try ebay.de or ebay.nl and use the search terms Mantelschoner or mantelschutz and rockschoner (coat and skirt saver or protector), they are available in transparent, but if the bike gets used, they scuff up on the inside quickly, messing up the transparent effect.