Made some I-clamps out of 19mm phenyl ply, based on a pattern for a C-clamp in Sport Aviation. The bottom horizontal bit sits under the form masonite, pressure is applied by sliding a wedge between the top horizontal and the longeron. Worked well. Thought for a while I had made twice as many as I needed, but will need all of them for the the lower longerons.
Cute Kira got sick of shaking off the off-cuts and let them pile up after a while. Tool I'm using is a jigsaw set in a Triton workbench, Australian man's final triumph over wood!