I used Sharon's method of using trim boards a couple of times, on two of the grandkids quilts. It works, my backs were totally smooth, which was a major issue I had. It does seem to take forever to baste them though.
Now, since DH made me the quilting frame, I only pin across the top while on the floor, then finish pinning up on the table, and it's good to go. So far all my backs have been totally smooth. The frame holds them snug.
I'm still waiting for someone to develope a way for me to just blink and they'll all be quilted.....
:-)