Just pack the inside of the box or screw hole(foot strap inserts) with regular old household flour. Pour it in and pack it down flush. In a box you can use a business card to level it off and take away the excess. When you cap your insert or lay up your Carbon Fiber the flour will soak up the excess resin. Once the lam has set just drill thru the hole with a next size smaller bit. On a box just use a Stanley knive to cut out the glass. Then grind lightly around the inside edge of the box with a die grinder and a Roloc disk. Blow or vacum the flour out of the hole. Like I said I used this trick alot on sailboard foot strap inserts because they couldn’t be masked. The reason we did a few finboxes that way was because it was easier to pack a Chinook box with flour than it was to mask it. It was also cleaner. If we routed a finbox or mastbox into Carbon Fiber or glass after the lam we would grind it down flush and then cap it with multiple cloth patches, Easier to fill with flour once than to tape and cut out .multiple layers of glass and tape. Lowel