I chose 6-32 T-Nuts to limit the size of the holes in the outsole, and because they are more than strong enough. I suggest you get truss head screws since the head has a bigger diameter and negates the need to add washers which I would recommend if you used typical machine screws with smaller heads. If you can get stainless steel screws and/or T-Nuts, that would be a plus to avoid rust but not a must.
I also felt that the pitch didn't match my ice holders so I added a shim on the heel as you can see as the white piece of plastic in this photo.