How to get a box call tone to roll over?

[PA-strutter]

I've made it to the point where I can get a high pitch out of my boxes but I can't get the roll over...any tips or tricks to get a long box to roll over on the yelp?

Also, what's the best way to "carve" or shape the sound boards? I've been using a chisel to shave wood away. I do put a small curve from the front to the back of the box but I've been leave the top of the sound boards "flat" as in not rounded off...

Any help would be greatly appreciated.

[culpeper]

If you can, post up a few pictures of the sound rails, profiles, top view etc., this will help a lot in order to comment.