Hello, I've asked so many questions and received so much help here I thought I'd post something that may help some people. It may not, but it's worth a shot.
I'm building my cab according to Saint's book, but the speakers I have won't mount the way the example does. The grill is only held on by one screw in the middle and the face of the speakers is slightly curved. So here's the solution I came up with.
I (well, my brother-in-law) routered three round-end slots in the speaker panel for each speaker. Then I added a wood strip to act as a ledge for the speakers to rest on. After that I rigged up a velcro strap system that holds them perfectly in place. Very snug.
Anyway, might be a decent alternative for speakers that don't lend themselves to the typical way of mounting. Plus if the speakers ever go out, they'd be a sinch to replace.