I usually force by hand an Allen key into the clamp gap, then twist the Allen key as needed to slightly wedge open the clamp.
I haven't left any unsightly marks/burrs on the stems I've installed this way, as the Allen key is not being forced into the gap end-wise but rather sideways, so pushes on a longer section of two of the Allen key's six corners.

Pictured below as I was subtly adjusting the clamping force onto an abrasive dowel as I opened up a 25.4mm clamp to more like 26mm