Going deep in the squat can be stressful for the knees over time, but a high box will alleviate most of the stress.
When using a high box, there's a tendency to increase the weight on the bar, but that can defeat the purpose. If you substitute the regular back squat for a high box squat because of knee issues, keep your ego in check to get the intended benefit.
There are several ways to do the high bar box squat. You don't have to use the crazy wide stance that's often used by Westside lifters, but wider than "regular" is often a good choice.
If you want to continue with your regular squat form, simply use the box as a measure of depth. Just touch the box; don't sit down on it fully like in a regular box squat.
Use a regular bar if your shoulders are fine, but if you have shoulder pain from squats or a lot of bench pressing, the safety bar is your best choice.
In the video I perform more of a touch and go version with a semi-wide stance. The chains are attached for loading the top portion of the movement while alleviating some of the weight in the bottom position.