I cut a larger hatch into my Escape
, an easy thing to do. Not sure if the curve of your Boler
would be a issue, or not, but I have seen others with them. All you need to do is to hold the hatch into place where it is to go, and trace around the inside rim (on masking tape put on the trailer). You can then cut the fiberglass with a jigsaw, Dremel tool, or something similar that will give you a fairly smooth cut. Then clean the edges up with a bit of sandpaper. Put the hatch back in place, and mark the mounting holes to drill. Put some butyl tape around the edge between the flange and the trailer, then rivet it into place.
Here is a link to a write up I did on the Escape