And they're done.
I knocked the corners off on the chop saw then used the shooting board with the 153:2 wedge to sneak up on the line. Then the 153:4 wedge was used to knock 1.5 degrees off the top.
Then the spandrels themselves were used to transfer knife lines onto the long aprons, and a router used to take the bulk of the waste off the bridles, and a chisel nibbled away at the rest.
There's a high risk that the chisel will overshoot the line, but I needn't have worried. Perfect fit on 8 spandrels.