I'm one of those weirdos that likes to cover everything. I use plastic caps on my grease fittings, sleeves or even pieces of rubber tubing over exposed bolt threads. Even heavy aluminum foil with cable ties if I can't find a better solution. Along with that, I always coat everything with anti-sieze.
I don't like wresting with parts that corrode after I've already worked on them.
So I maybe go a little overboard.
Anyhow, what I would do is get a suitably-sized rubber bellows. You can then cover the u-joints and give a clean appearance. Could probably use hose clamps to secure them, although there are probably cleaner-looking ways. Even cable ties might work.
They'll keep out dirt and grime and look decent too.
McMaster Carr typically has all sorts of stuff like that. (www.mcmaster.com
) although there are probably cheaper places.