For cleaning up parts post-printing with an X-acto knife I often am holding the handle in my four fingers and using my thumb to push the part against the blade, this allows for finer control when cleaning (i.e., cleaning brims) and usually I can control it enough that the blade never hits my thumb, but sometimes the blade is too dull to cut cleanly through and jerks or the part slips so I designed a thumb thimble as extra protection.
This is sized to my thumb so may need to be scaled for your thumb, the inner oval is 19.5mm x 26mm (diameter in two directions). The cut outs on the bottom were to make it easier to move around my knuckle.
I didn't use supports because the overhangs were not too bad and I didn't want to have to try to dig the supports out all the way up inside the thumb. I did use a large brim (10 outlines wide) to make sure with the small contact surface it didn't fall over or come off the bed. A raft would work as well but just use more filament and time.
Forming to your thumb
I did have to apply a little heat to not only smooth the edges but also I put my thumb in while hot to make it form better. CAUTION depending on the filament the temps needed to heat the thimble enough to form can severely burn you. I suggest PLA due to it's low melting temps and maybe using a thin latex glove or something for added protection and frequently dipping your thumb in cold water to keep it from burning.
I made this quick and easy just using TinkerCad, just a few simple shapes, nothing fancy.