It is unfortunate that the cabinet is beveled on the bottom edge. The lack of clamping area is the thing that stands out to me.
Have you played with finding the center of gravity for the speakers?
Determine the tilt angle you would like to use.
I would guess if the front of the clamp was under the front edge of the bottom, they would want to fall back into an upright position instead of falling forward.
You can do experiments using the edge of a table or counter as the front edge of the support plate. While playing with the overhang at the front, just balance the speaker at the desired angle and see if it wants to fall forward or backwards.
If you can position it far enough back on the clamp that it wants to fall backwards, you really don't have much of an issue.
Your current solution is very secure (as in overkill
), but I can understand that you might not like the look of the cable wrapped around the support. Black speaker wire is also an option.
