A number of factors must be taken into account when buying a Kitty Fence. You should consider the setting, your budget, the cats’ potential for containment, and your risk tolerance. DIY with hardware store supplies may be your best option if your budget is low, your risk tolerance (your ability to tolerate failure) is high, and you have the time to experiment. You may want to consider one of the more basic commercially available cat topper systems if your budget is low and your risk tolerance is high, but you do not have a lot of time to spare. When fencing is not available, you have additional factors to consider, such as whether you do it yourself (DIY) or have it installed by a contractor such as https://www.critterfence.com/cat-fence.
Purchasing a commercially available cat-proof fence product/system (from a cap-only or from top-to-bottom) will require you to take into account your own needs and expectations. The simplest fence topper system is probably the best choice if your fence needs are simple (say a 6 ft wood privacy fence without trees, sheds, etc. nearby) and your risk tolerance for failure is high and/or your cats are not very competent or motivated.
Along those same lines, if you need a ground-up fence and have a very simple layout (rectangle) with no trees and your risk tolerance for failure is high and/or your cats are not very capable or motivated, one of the more basic ground-up commercially available systems may be right for you. If your needs are complex, and/or your cats are capable, and/or motivated, and you want a proven system that works in the widest array of scenarios on even the most talented escaping kitties, you really only have one choice (shameless plug shrouded in honesty.)
Install a cat-proof fence roller on an existing fence:
Additionally, you may want to consider using a cat-proof fence roller to modify an existing fence. Coyote rollers are repurposed from cat fence top rollers. Coyote rollers are cylindrical apparatuses (like cylinders) between 1.5 and 3 inches in diameter that are placed just above the fence and spin. Imagine running on a log in the water as a comparison.
There are more animals jumping up and clambering over fences than digging into them. Instead of a solid top to the fence, they encounter something that spins and they can’t hold on to. Losing momentum, they tumble. However, DIY online instructions are subject to experimentation/trial and error, as well as the most basic of instructions.
As opposed to standard cat fence toppings, the rollers used by commercially available fences are less visible. There are even a couple of commercially available variations, including one that has an offset to the inside of the fence and one that has fins instead of a solid cylinder, both designed to be more cat-friendly. All commercially available roller tops for cats are more expensive than traditional fence topper systems, and the specialty ones are much more so.