I let them sit for a day while I thought it out.
Then, I went slow. I dealt with the depth first, taking a fillet out of the back side.
Once I got the depth close, the sides were that much longer.
Wedges taken out here & there, long slices in other places & then glued back together.
The seat backs in my case were the worst. Cut / glue, test fit cover, take more out, test fit cover again & so on, etc.
The first one required two extra hours, the second one about 45 minutes because I took good mental notes on the first one.
They came out great though.