After developing a good set of calluses on my fingers while learning to play the banjo, life gets in the way, I inevitably lose them, and need to start all the way back at Step 1.
This simple device allows me to press my fingertips onto the facsimile strings a model having the dimensions of the 2nd and 3rd frets of my 5-string banjo. I use it during my long commute to and from work and thankfully, over summer vacations that don't include the instrument.
It doesn't endow me with more musical ability, but at least my fingers hurt much less while playing the real thing.
I made measurements of the dimensions of my actual 5-string banjo and created this simple design with Tinkercad.