I think the B/E&A repop cover (link below) will fit and seal on an aftermarket master cylinder (as in picture), but wanted to confirm.

http://www.beaparts.com/list_items.aspx?body=B&cat=E&subcat=41

The aftermarket MC uses a 5/16 inch bolt to hold the cover on instead of a 1/4 inch bolt so I figure I can just enlarge the hole in the B/E&A cover.

Conveniently the original 1/4 H head bolt is hard to find, but a carb bolt looks just like the original, but is 5/16".

I have both style masters and I was checking the same thing as you and my answer is no.
The reason is I think the caps will fit each master BUT the 1/4" bolt will not give you enough room to tighten it on the cap designed for the 5/16" bolt. The camel hump bracket is bent for a 5/16" bolt to fit in the valley so the head on the 1/4" bolt won't let you get a socket or wrench on it to tighten it down.

My memory isn't what it used to be but I believe this to be true.
I checked this out about a year ago.
David