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> I'm not quite sure how to make one that will be as resistant to impact or scrapes as an engineered one.

Many large lenses are sold with fabric "socks" as their only protection, so you may be worrying unnecessarily.

I personally have a Sigma 300mm f/2.8 which I acquired second-hand with no cap or cover at all, and for which I made my own. I used a circle of hard plastic, about 1mm thick\*, covered with two layers on the inside and one on the outside of black foam from the craft section of a cheap department store, which I stitched into place; and then side walls of the same foam, again stitched. It took about 4 hours to assemble, fits very snugly\*\*, and should certainly protect against scratches and light jostling. (I will add that in an abundance of caution I left the assembled product to air for a couple of weeks before using it, just in case the foam was outgassing anything which might harm the outer coating of the front element).

\* If you ever bought CD-Rs, they came in a spindle with a piece of plastic the same size as a CD, presumably to protect the unused ones. It was the perfect size for my lens: you would obviously want something larger.

\*\* I had intended to fold back the top and make a channel for a drawstring, but that wasn't necessary.