I recall a picture or diagram of the tool for measuring
surface deflection. It may have been in one of Dick
Johnsons articles which are or were available at the
SSA web site. Iif not it was written up in Soaring.

Use a small block of alu or plastic.
Dimension are 2" Long 1"wide 1/2 thick.
Use three 3/16" round head Nylon machine screws.
Install two screws at each corner and one at the middle on the other end.
Install dial indicator by drilling a hole in the centre of block
as well as cutting a slot for clamping the dial indicator with a screw
or look up 1989 Soaring page 35.
That was a year after I started flying gliders.
Regards
Udo