Just spotted this if you haven't sorted your issue yet.
Looks like these work by using the actual accessory's clamp, allowing a reduced depth by not requiring clamp bolts in the spacer itself; just a slit to allow movement, similar to what I described earlier.
[i have no connection with the seller]
"Alloy construction", huh? Hmm, aluminium alloy, ferrous alloy, titanium alloy...