yup, that looks good to me. You might find that you need to cut the bolt down (too long, possibly), but it's really difficult to measure the exact size you need before you've got something you can stick in to measure. A junior hacksaw would work fine for this. Preferable is to measure the length you need (offer the speaker up, put a bolt/screw through and mark at the front where the screw is in line with the wood. Add a few mm on to this just to be safe :) ).
I didn't bother yet, so my 2 new speakers have 4 bolts each sticking out about 1cm from the front of the cab! Never mind, eh :)
Roo