Step 4: Rock on!
My accuracy is better with this abomination. Says something about the engineering quality of the default pedal, I suppose. If you're having trouble, make sure the switch is aligned with the magnet. You can also find square mallets at a well-stocked drum store, could always get one of those, and extra magnets, to give yourself a better strike zone.
Padding between the mallet and the project board wouldn't hurt, but don't use anything too thick or magnetically-shielding, or your switch won't trigger.
Also wouldn't be a bad thing to use better materials...I'd have attached this to plywood, if I had any scrap laying around. In that case, you'd certainly want some softer materials wrapped around your magnet, or you'll eventually crack it smacking into the contact zone.
The foam board seems fine, but I recommend at least a double layer so it doesn't snap in the middle of your first song. Of course, if it does snap....duct tape can totally fix that.
You could also put a rubber pad above or below the magnet, as long as the pad is a little thicker than the magnet, so the pad will hit the striking surface, and the magnet will just be close to the surface, tiny bit of air-gap to reduce direct physical impact to the magnet itself.
I've included one last view from the back, since I'd already uploaded it.
Now...Go play your drums.