Originally Posted by krazygluon
you might also put a piece of fabric (or maybe a product exists for this) against the sidewall gash to add some structure there.
To quote Sheldon brown:
Boot
A temporary patch to cover a large hole or cut in a tire. A boot does not need to be air-tight, but needs to be sufficiently strong and stiff to prevent the inner tube from bulging through the hole in the tire.
Many things may be used to boot tires, including leather, currency, food wrappers, etc. Patches intended for tube patching will not usually do a good job, because they are stretchy and will blow through the hole just as an inner tube would.
The best material for boots is a piece cut from an old thin-tread tire, preferably a tubular. Many cyclists carry such a piece of tire for this purpose.
A boot does not need to be glued in place. It will be held in position by the pressure of the inner tube.
Bolt-on