I have the edge of my belt mounts set in about 1/8" from the edge, with the screws perhaps 1/2" in from the edge.
I have blocking underneath to ensure there is adequate support for the screws to bite into.
This was part of the original design, so the blocking was taken out of scrap from the original frame and all four corners are identical- but it is not critical.
The screws I used are #8 x 1-1/2" Black Truss Head Wood Screws.