Hi Dan,
Can you post a link to the page for purchasing them please? Mine was in their catalog and is dead/moved now.
I skip the corners and just use the Nielsen type (I have loads of them). I also don't use cleats, just a wire hanger from the little screw in side pieces that come with Nielsen frames
Lastly, if mounted on Dibond or equivalent make sure you peel the plastic off the back before you stick the subframe on. Don't ask me how I know it's a bad idea if you don't.