To build one's own replacement / fixing parts is one of the great advantages of having 3D-design and 3D-printing at one's fingertips.
My problem was that on my trusted old (1998) GIANT ATX 870 mountain bike, a custom friction shifter (as opposed to the more recently popular indexed slot shifter, where preset positions for the shifter are supposed to correspond to the front derailleur position over any of the three chain rings) would not stop to shift laterally, towards the right pedal, leading to the occasional drop of the chain. Friction shifting has a lot going for it however, particularly as I grew up riding to school with friction shifting. One can listen to slight noise differences and adjust the levers to a position that is always optimal, without need to readjust the shifters all the time, and without need to over service the bike.
That chain drop should be avoided from my view, so I figured out a fix.
The original chain guard would not prevent a lateral / pedal sided chain drop. And a few years ago, that original chain guard part disintegrated by itself, probably due to UV-light damage accumulation. So I removed it but kept the screws.
I then designed a chain guard that would be as close to the chain and the derailleur as possible. The final result is perfect.
Here is a photo of the final design getting printed (MakerBot Replicator+ 5 Gen, Tough PLA, almost 100% density).
The chain guard gets very close to the front derailleur and definitely exceeds the height of the chain sitting on the largest chainring.
I had considered other colors, but went with the dark gray Tough PLA first. I did provide for other screw types as well, with extra space for washers, but turned out the original screws kept that thing in place like super fast.
Easy to see how the chain now is prevented from coming off too easily when accidentally pushed in the wrong direction with the unconstrained friction shifter.
The thing in action after mounting it.
You can get your own chainring cover made, either by downloading the model as STL and printing it yourself, or, by ordering a printed part online to get it made and shipped to you.