I tried a foot switch a couple of times and found them to be impossible to work with, I move around the easel when dodging and burning and never could find the damm thing.
Also another trick ,, make a big card black on bottom white on top and cut out a big hole, you will be able to see the image as you burn.
Now make a series of smaller cards like the bigger one with smaller holes Then you can lay one on top of the other and for big areas use the big tool and smaller areas use the small hole card on top of the bigger one . This allows you to not flash the outside areas but also get into really small areas.
I had the same problem and ended up mouting the footswitch on the side of the enlarger cabinet and I hit it with my knee to turn it on. And I am going to adopt Bob's "nesting burning cards" strategy!