Perhaps a pulley mounted to a 2 x 12 of sufficient length that the
glider sits on the board to hold it down. Position the glider on the
board to get the desired angle and put a couple of people in the
cockpit. Use a spring scale and come along to set the desired tension.

snip

We're thinking along those lines too. However, to check a CG hook at the rope angles normally experienced, the pull angle has to be something line 45 degrees down from horizontal. That's going to be complicated.

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Dan Marotta