I've had arthritis in my left thumb for a couple of years and have been wearing a brace off an on depending on the pain and many times I just wear it at night which seems to help.  I did a 3d scan of my left hand and created this in Fusion 360.  It fits well and and is comfortable. I've included a mirror image for the right side.  I CA'd velcro strips together to make a comfortable strap.  

You could heat it up with a heat gun and mold it to fit better if needed.

Print with support as shown in one image.

Hope this is useful for someone, you'll have to be the judge if it's helpful.

Cheers

Kisssys