I wanted to cover the terminals on some lead-acid batteries while they were not in use. I made this model and first tried printing it using ABS. It works, but its difficult to remove.
Then I tried NinjaFlex filament. Other than the print being a little uneven and "grainy" (most likely due to my print settings--still new to using flexible filament), they work great. They're easy to attach and remove.
The "columns" on two of the outside corners were an attempt to add some texture for gripping when I was experimenting with ABS. They became unnecessary when using flexible filament, but I left them in the model.
Note: these are for the larger F2 (Faston tab #250) terminals, which are .25" wide, but they will also fit the narrower (3/16") F1 tabs. Both sizes are the same thickness so they shouldn't be prone to falling off the F1 tabs.