I've tried many things, and the best method for me is to simply apply patches with gaffer''s tape.

But what you want to do is apply a thin layer of contact cement on the bellows before applying the gaffer's tape. Contact cement will bond but remain pliable, and gaffer's tape won't leave any residue like you might get with other types of tape.

I've tried liquid electrical tape -- worked great for automotive work but wasn't so great with bellows. My exerpience was a sticky mess.

By the way, if that Speedex has plastic bellows, you'll be patching that thing forever. Your best bet is to find a Speedex with leather bellows.