It is easy to construct being essentially an upside down periscope such as I used to make when I was a kid.
I have no doubt the commercial system uses better quality mirrors etc. - but this is a lot cheaper . Of course I don't suggest you go into competition with the original manufacturer, in fact this is only posted as a matter of interest to show how these things could work.
Step 1: View of headup display in use
Step 2: Over view of the assembly
Step 4: Side View
The size depends on your monitor and web cam sizes.