My son and I do a lot of indoor rock climbing, which can lead to very strong finger flexor muscles that are not balanced by extensor muscles. Since we recently got a 3D printer, I designed a simple printable finger muscle excerciser (it needs one or two rubber bands) that lets fingers get independent extensor exercise.
Note: Before using this for any rehabilitation or medical purposes consult your doctor or therapist.
Step 1: Print and Cement
Get the stl file from the project's Thingiverse page. If you want, you can use their Customizer to adjust the dimensions to fit. I designed it in OpenSCAD so it's fully customizable (I'm new to 3D design and have found OpenSCAD much more intuitive than GUI tools, maybe because I'm a programmer and mathematician).
Ideally, print in ABS (you can also print in PLA and join using screws and nuts, which you'll have to take apart to change out rubber bands). No support needed. If you have a smaller print bed, use the Customizer to break it up into a top print and a bottom print.
You'll see a big central circular depression on one half and a post that fits into it on the other. Cement these together, being careful not to glue any other posts together--they need to be able to be flexed apart to change the rubber band. I used ABS juice as my glue--ABS dissolved in acetone. You can also use screw and nut if you prefer. You want the bond very solid, so let it set for several hours to be safe.
Step 2: Insert Rubber Band(s)
Once the cement is set, put a rubber band around the edges. Then carefully bend back the corners of the exerciser to let you put the rubber band (red in drawing) between the outer and inner posts, one by one. If you opted to screw the halves together, you won't have to do any bending: just open it up and insert a rubber band.
You may find your thumb needs more tension. Put a small second rubber band around the outer thumb posts (blue in drawing). I ended up looping that twice.
Insert fingers within rubber band, and stretch, repeating.