OK - here are the specifics:
You need 1/4" X 20 thread. I suggest cap screws in stainless steel. Cap screws are allen head. Get 1/4" flat washers. Home Depot carries all of this as regular stock. It will cost less than $5.
To determine max length, take a wire and place it through the binding until it bottoms out in the hole. Measure the distance. Go 1/4" shorter than that measurement and the screw will not bottom out and damage your board. It is OK to use a few washers to shorten the distance. Also, while in Home Depot, go to the plumbing aisle and buy a tube of pipe thread joint compound. This will keep the screw from seizing in the hole. It will not harm the board. The tube should cost about $2 and will last at least a year. You will also need a 3/16 allen wrench to tighten it down snug, not tight.
If you have more questions,
rider@boarditup.com