I don't have a humbler, although it is on the to to list. Here is a ball crusher to give some idea where to start with the opening dimensions.
You can make adjustments for your own needs. As Ponylady mentioned in her post, since you are building a custom model, most of the measurements will come from you.
As a note, take care of the pinch factor while tightening
Might help to build a couple short sample blocks and try them out.
For the final product, use a strong piece of lumber with the grain in the proper direction. You would not want it to break at the weak point in the center with the sensitive bits in between.
In the attached image, the opening is 2 inches by 5/8 inch at the widest points.
The graph paper is 1/4 inch squares for reference (take into account the camera perspective)
This was a first pass for me and I believe the opening could be a bit smaller in the short dimension, Maybe a 1/2 inch or even 3/8?
(Someday I'll finish it with some stain and a water based polyurethane finish.)