Here's some different setups that I've seen:
-Full length copper pipe that had rectangle cutouts to slip over sights, held on by Velcro straps
-PVC split with table saw, rattle can flat black, slip'd over barrel flats, Velcro again. length varied by the shooter
-Flat steel stock formed into a U held on with various means (elec tape, leather thongs, kite string) length=6-9" each to full length, your choice
-Globe sight hoods, like the Sharps
Now, I am too cheap to buy fancy materials, so I fashioned mine from two soup cans, already round, light weight, just enough tension after split lengthwise to hold position, and I find them to be of sufficient length to cover for shade yet let in light to see. I tie these on for security in the wind and not be distracting blowing over onto the next shooter.
Everyone has a different perspective of how long they need to be. One of the biggest goals is to keep the glare off the sights. I find that it also trains the eye to focus on what is in the tunnel.
I was at a chunk match that was overcast, using my shaders and had a guy tell me " you won't need those today". I left them on and won. Needless to say he was *#)*^~ off. Maybe it was because my rifle wasn't finished. It was still pretty square plank. I had roughed in a cheek piece and layed duct tape over the rasp marks to keep from getting splinters in my jaw.