We also use the foam pipe covers ...less than $2 at Home Depot:
Tubular Foam Pipe Insulation
They are good for protecting the rubber from deteriorating in the summer sun and also good in the winter. We actually use them in the winter on our car on frosty nights so the wipers aren't frozen to the windshield in the morning.
If you absolutely want to full covers, I'm thinking you can still use a 28" cover on a 24" blade. It may be a sloppier fit but will still protect your entire blade and that's what it's all about.