Im looking to make some fork covers/protectors like in the image below but my forks have a slight taper on them.

I'm not sure what bike you have there, but I love the triple clamps, look awesome. Agreed that a vertical slit would be the way to go. I will also add that they should be stuck on so as not to rattle and cause damage to your fork tubes. Something that will be thin and isolate the fork protectors from the fork tubes would be wise, I'd recommend silicon adhesive that is non acidic as this will eat into the aluminum. If you're not fond of having a the slit opened larger at the top due to the taper then perhaps best to make a tapered mold/plug by machining something on a lathe - some hardwood that could be coated for a better finish perhaps. Just an idea. Will be great to see how these come along, I'll probably make some for my bike.