These guys are getting close.

Create blocks that you can clamp to each apron. One face of the block
is parallel to the apron, the other face of the block is parallel to
the face of the leg. Then clamp across the blocks. If you place the
blocks far enough back along the apron you can get the clamping force
directly lined up over the spline. Place a clamp above the whole mess
and is you have a clamp with a deep enough throat, place one outsode
the leg too.

The blocks will be sorta shaped like this \__ with the inside
angle of 135.