What I am using is a piece of thicker paper or similar plastic and then laminate it with a normal laminating machine.
You can easily integrate a hole or long-hole for the bolt of the splinter guard.
The thing is quite stiff and the last millimeters of gap are closed by the vac-suction.
I thought about getting something printed, but for now my low-tech version works brilliantly, so I have not been inclined to invest more time/money.
Basically this was born from a leftover off-cut when I made my own Systainer labels. [emoji41]