This is something I meant to add years ago but forgot due to it not being mentioned often. Get an assorted spacer collection also known as "stand offs" (basically thick washers) to put on the two bolts between the clamps to keep the clamps parallel to each other. This will ensure you get even clamping pressure across the penis shaft.
If you ever bought a wall mount for a flat screen TV, they always come in the package with the bolts so you can space your TV off the mount where needed.
They come in assorted lengths so they will cover everyones girth and you can get a few sizes for under $10 or a larger assortment. You can get short ones and double them up or get and assorted set with different lengths.
Just make sure the inside dimension of the spacer is at least 1/4" or M8 so they slide over the bolts something like these...
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Maybe something we can add to the package?