Number of active soaring pilots in the US?

I am trying to get a handle on the number of active soaring pilots in
the US for some statistical studies. In 2003 John Roake from New
Zeeland stated in a table he developed that the number was 29390. He
also stated that the FAA and SSA numbers were not representative. I am
looking for ideas on how to firm up these numbers so they make sense.
The SSA membership numbers were less than half the developed numbers.
I know that you don't have to be a member of the SSA to be a soaring
pilot in the US but to claim that half the soaring pilots in the US do
not fly at established sites seems a fair stretch. Any ideas?

Bob