instead of repairing small cracks on chimney crown has anyone put on a chimney chase cover?

I made a few cap covers when I was roofing. They look nice and work very well.
We hand made ours, not so bad when you have the equipment to do so.
The copper ones really look nice.
You can try the pre made ones, or have a local sheet metal shop make it for ya.
No more sealing or worrying about cracks.

My only comment would be go with stainless for sure.. our galvanized one started looking bad in 5 years, and leaking through pin holes in 8.. gooped up with roofing silicone now.. plan on a SS one soon.

About 15 years ago I repaired a friends chiminey crown and it still looks good today. The old one was cracked and had turned loose from the brick. Prep was easy as most of the crown had already turned loose. The old crown went over the edge in pieces, sastifying work.

After clean up and a splash of water to slow the loss of water from the mix I applied the following. One standard bag of sack create mixed with water and about a pint of carpenters wood glue. Mixed to a stiff consistancy and troweled into place. Note: the glue is not a one to one replacement for water. You could use the "moose milk" that is made for that application but the glue does the same or better and is cheaper.

Having said that stainless steal is hard to beat for looks and durrability.

Mine is not copper or a professional job by any means but I took some 4" sheet metal angle and fit it to go around the outside of the chimney then used flat sheet metal for the top. I have a hand made chimney and nothing store bought was going to fit. I just used some tapcon screws in a few spots to hold it down. Looks pretty good from 30' down on the ground (kinda like a 55 mph paint job) but it works and it keeps snow and ice from sitting on the crown and rotting the mortar away.

Crown Coat from Chimney Saver is the only product I would recommend. Unless you your chimney is really small, you will most likely need the 2 gallon container and it runs closer to $250! It has a 10 year warranty.

I have put SS chase covers on several masonry chimneys. It is a great idea as long as it looks good with the house. I opted to cast a new cement crown with an overhang instead. It only cost about $45, otherwise I would have done SS.

Chimney Rx makes a line of masonry products that are sold in my local area at masonry supply houses. They have products that are similar to the Chimney Saver line. The Chimney Saver warranty is only applicable when their products are "Professionally Installed" which for me did justify the extra cost for diy. The Chimney Rx brushable Crown sealer does an excellent job as does their masonry water proof sealer which is siloxane based and vapor permeable. It also does not alter the brick and mortar appearance. The crown sealer is available as brush able and trowel grade and runs about 50.00 a gallon 98.00 2 gallons, the brick sealer is about 25.00 a gallon.
The best way to do it is as webby suggested and form and pour a concrete crown, however virtually no house in my area is done this way no matter the age. My house was built in the 50's and has had a mortar crown since day one. It has been re mortared twice since it was built. I just removed and re mortared it a month ago, and coated it with the Chimney Rx sealer and sealed the brick. It looks like the day it was built, nice stuff

NOTE: Just did a little more research and it seems that the Chimney Saver and Chimney Rx are indeed the same products made by the same company Saver Systems in Richmond Indiana. The only difference seems to be the chimney professional line is the more expensive longer warranted line. The Chimney Rx line is sold by masonry supply houses and company's like Northline Express for considerably less money.