This is a simple finger extensor muscle exerciser. It may be useful for rock climbers for better balance between extensor and flexor muscles (I've seen claims that such a balance can help prevent tendonitis).
Print the top and bottom, and glue (or screw) together at the big central circle--I printed in ABS and using ABS juice to glue together. Then you can put a rubber band between the pairs of posts simply by bending the exerciser a little bit. You may need to loop the rubber band additional times to get the right tension, and I ended up putting a second rubber band for the thumb. Adjust tension between fingers as needed. When the band breaks, just put in another.
Edit the dimensions to fit your hand.
This is not a medical device. Do not use for therapeutic purposes without consulting a doctor or a licensed therapist.