Legal and correct is good but that does not imply TSO. The "Landings"
site has a good FAR search engine. Go to

http://www.landings.com/evird.acgi$pass*76615480!_h-www.landings.com/_landings/pages/search/search_fars.html

and enter TSO for search and 91 for the part to search. Unless your
customer plans to operate other than under part 91.

I think you'll find that the only TSO requirements related to gliders
operating under CFR part 91 in US relate to transponders and ELTs.

Andy