Glad you are making angled adapters. I would suggest a 20 and 25 degree option as well.
These extensions are not like many of the other one piece extensions that have a built in rise of 5-10 degrees, so in order to get to a more angled position you really need a 20+ degree wedge.
Having two sets of holes is a good idea! Most only have one.
For what it's worth , I use the Fouriers 15 degree horizontal wedge and able to get 3 crews into mounting holes. That 3rd screw REALLY made the mount much more solid.