I wanted to suggest Sugru too, but
-) it needs investment (something David sure wants to avoid)
-) it is a rubber compound, which may not be benefitial at this part (I do not know of any rubber transport lever), and I am not sure how well it would stand, how resilient it is (I never had Sugru in hand).
Aside of the grinded piece of hard plastic or hardwood, the at once solution would be a piece of PVC or rubber tube of apt diameters. The PVC tube likely has to be heated to adapt.
Or textile adhesive tape, but David already hinted at such himself.
Or... just grinding the edges and corners of the lever to yield some comfort (I do not know the lever in question).
I agree - I've not played with it because it's expensive, even though I can afford to drop $20 - $30 into the garbage. Maybe some day I'll have a reason to try it out.
David, what is your priority? Something functional, I am sure, but how decent do you want it to look? Merely nice, or like it was made for the camera?
You could probably snip and rework an old 35mm canister to make a thumb-rest, drill a few holes, and attach it to the lever. You could use screws from a discarded computer hard-drive (though would need Torx bits to unscrew them).
Staying with the discarded hard-drive idea, you might be able to adapt the internal actuator arm to be a new lever. It believe it's a non-magnetic metal that can be filed and drilled to fit (I want to say it's aluminum, but am unsure). It would not have a plastic thumb-guard, but should be smooth enough to be comfortable.
I replaced a small rubber "roller" in a Super 8 projector with a piece of vacuum hose, but it's internal and no one has to look at it.
Another slip-on idea would be a "vacuum cap," like you would find at an automotive parts store, though I don't know how easily the lever would eventually cut through it.
https://shop.advanceautoparts.com/c4/vacuum-caps/-49996419
Looking online, it seems the lever is thick enough to not bother your thumb, and that the plastic attachment gives a small but needed extension to the lever.