Sometimes things just work out. A friend of mine has a fiberglass radar arch on a 28ft Monterey, and he was not happy with the sound. Replacing the inexpensive stock speakers with Infinity replacements did not improve the sound as much as he had hoped, Then one day, he inadvertantly removed the inspection plate from the arch, and the speakers sounded better. He inadvertantly created a vented enclosure when he removed the inspection plate; either a Bass Reflex or perhaps a Horn enclosure.
Whether or not the speakers are technically Bass-Reflex vs. Horn, the fact is that venting those speakers allowed them to have increased bass. In these situations you don't really want to know why things happen, but you should nevertheless take advantage of the situation.
The decision then became, how to make this improvement permanent? Inexpensive metal grills can be purchased from Madisound or Parts Express as well as others, in many different sizes. The two piece construction works well for this project as the base can be attached to the inspection port opening, then the grill can be snapped over it. This also allows use of the inspection port for it's intended purpose. Generally the ports are only available in black, and a bit of Krylon paint solves that problem. There is no need to seal the base of the grill.
The back grill is screwed to the inspection port. The grill size was chosen to fit into the indentation on the arch. Then the grill cover is simply snapped into place.
This of course will not work in every situation, but when things are just right, it can be an improvement. So if you have an arch of this type, try it and see what happens.