Cable Release mount too large

[Trooper]

I have a Horseman 980 that I would like to use with a cable release. The basic cable release from Adorama is too small to fit in the opening which appears to be 6.5mm. There is a screw that will tighten to lock a cable release in place, but it doesn't even touch the cable when I insert it.

I've noticed that Horseman makes a cable release, but I'm not willing to pay that much unless I know it will fit. What do I need to make a cable release fit?