There is another option.. did it on one a few years ago where we made the mast
a tabernackle to go under the bridge in Santa Cruz
.. fit the plate to the mast
and then went to a truck shop (peterbuilt) and had them install "Huck Nuts" in the mast with their expensive tool.. it sets a crimpted nut into the mast with about 1/2 inch of threads, much like a "Molly" set for a home wall..
Your 1/4 inch screw then screws in.. I used stainless screws with alen heads and plenty of "never sieze"