This is how I adapted my mountain bike.
I first got in touch with the British paralympics cycling team who then put me in touch with European gold medal winner on the indoor track (who’s name I’ve forgotten.) He did not live too far from me. He kindly invited me to his house in Derby to see how he had adapted his bikes. While I was there he also showed me all his medals and trophies he had won over the years. He has quite an impressive collection.
Basically all I did was to buy a pair of cheap metal brake levers and then the bracket off one of them I cut in half then had it welded piggyback style onto the other one so both levers can now be pulled by my one hand.
The gears where also easy, I just used a thumb and finger gear change, as well as a twist grip and mounted them on the same side of the handle bars as the double brakes.
I also made a hand brace for my B.P.I. hand out of two wrist supports. I took the metal strip out of one, bent it over at the end into a hook and then pop riveted onto the metal strip in the other wrist support. I have included photos of both my mountain bike handlebars and of my wrist support.