I prefer using Toggler toggle anchors.
They can be adjusted up to over 3 inches for thick walls and are extremely strong. They are overkill for a magnetic knife bar but will work nicely.
The issue you will likely have is the size of the head of the machine screws that come with it will be too big for the keyholes you have used on your knife holder. You may have to find some with the same thread with smaller heads or file/grind them down or modify your holder with larger keyhole brackets.You will want to get a nut and washer to fit each screw since you want the head to be offset from the wall for your keyhole bracket. Thread the nut/washer all the way to the head of the screw , thread the screw into the anchor until you have the head offset to the distance you need for the keyhole and then tighten the nut/washer down to the wall there by pulling the toggle firm to the back of the wall. (might take a few try's)
They require a 1/2" drill bit to install. You drill the hole, adjust the anchor so you slide it through the hole and adjust it back to perpendicular and pull it tight to the back of the wall. While pulling on it holding it tight to the back of the wall you slide the tab tight to the front of the wall, i use the shaft of a screwdriver to force the tab to the wall while pulling on the plastic pieces. Once the tab on the front is holding the toggle tight to the back of the wall you snap off the extra plastic bits and you are ready to thread in your screw. Once you have them in your hands you will understand the mechanics of them.
The are available in two packs i believe.
Trevor's option will also work. You may need to bend the little spike thingy's back that are designed to punch into Sheetrock as they will not easily penetrate the airstone.