I made a few adjustments for a mount for a friend.
He did not have as such exposed seat rail length as I did, so I made the following modifications:
Reduced the width of the top piece to 3/4" due to the smaller length of seat rail available to clamp onto.
Added 1/8" x 3/4" wide aluminum flat stock on top of top piece to distribute load and prevent Corian from cracking. Filed the drill hole to a square shape to accommodate the use of a carriage bolt for the clamp bolt
Used double-sided tape to secure flat stock to top piece. Let some of the tape overlap the drilled hole to help hold clamp bolt when not secured.
Also, I tried a piece of Avonite instead of Corian.
This particular Avonite selection had a course granular composition which made it look like quartz. This product did not seem to machine as well, for this particular application. I would stick with solid surfacing products that have a much finer grain.