That's a nice setup you have there, the zebras look great too.
You will probably find that the zebras will not occupy the clay type caves that are raised so you may want to place them at "ground" level at some point.
Like chanettt said, if the slate caves are too big then they will prefer the clay ones. most members here have slate caves, but we've seen several spawns in clay caves as well.
You'll just have to see which ones the fish prefer
have a look here. http://www.zebrapleco.com/forum/viewtopic.php?t=2749 for a good type of diy caves. The dimentions should be 1,5 times the fish' length, height the fish' height + half dorsal fin, width the fish plus one pectoral fin.
The zebras are not likely to have a preference for either type of cave, it's the dimensions that are important. Most people use slate because you can make those up yourself quite easily. The clay type caves need to be baked in an oven and I'm not sure if a domestic oven will get to the temps required. I actually quite like the look of the clay caves, they might actually look fairly natural once algae starts growing on them.
