Ron,
I think you are on the right track using epoxy. I am not at home right now, but I will get back to you this evening with specifics. There are several epoxy brands that will work. However, they all have slightly different characteristics. Those that mean the most to me are: being self leveling, thin type, slow setting, and water clear (color). For your purposes, slightly different characteristics are desired. I will get into those later. Stay tuned...
Larry
Ron,
I use a 2 part epoxy which has a 2-1 mix ratio. Raka 127/350.
http://www.raka.com/epoxy_kits.html There are many epoxies available, and I have used several, but this particular one exhibits all that I am looking for at a reasonable cost.
It's a "No Blush" type epoxy that is just right in hardness. It levels well, and does not leave air bubbles. The slow hardener is an asset, as it gives you more time to work before it sets. You could apply 3 coats in one day, and sand the next. But.....
I prefer to apply 1 coat and wait about 12 hours, sand lightly, and repeat till I am happy with build finish. Epoxy builds thickness very quickly. Light sanding (or scraping) between coats knocks down the highs before the next coat. The final coat of epoxy (#3or4) will look almost as smooth as the final paint finish. This hard epoxy base will require a lot less paint to finish your speakers.
If need more info/tips, ask here or PM me.
Larry