It should be pretty easy to get a piece of scrap mast extrusion and cut a scab that can be riveted over the whole swiss cheese area. You will probaly need to use a body hammer to adjust the shape for a tight and conforming fit on the curved beam (we are working on an H16 aren't we?).

Once formed it can bet set in epoxy and riveted in the corners. The new mast base can then be mounted using the conventional procedure.

Having said that, a used beam would seem to be the best and easiest solution.