You can not fix worn boots. You will never get anything but a temporary results. Most boots from this era bike needs replacement. Heat causes them to loose pliability and they come loose as yours have. They should still be available, Suzuki still makes them for my '78 GS1000E. Order clamps when you order boots, they stretch over time and will not clamp properly.
The rubber part of the boot is still very pliable, and only 2 came apart as I
pulled the carbs out, so if I can find out how they are sealed onto the metal flange I think they would be serviceable. Anyone tried this ?
I have not heard of anything that works for long. I'm glad to hear about the softening technique but I'm not convinced it wouldn't ruin all of the Suzuki intake boots that are molded to metal flanges. It sounds like it would work fine on the air box side. Most all of the members at (http://www.thegsresources.com/) replace the intake boots. Many initially try to fix them but eventually end up replacing them. I would like to find a fix for the old sets I have, they are not cheap.
The flanges are smooth and tapered so just molding without any adhesive I wouldn't think would hold. Anyway I will research adhesives and see if I can
find one that will work. Let you know how it turns out