Don't do it. There are other options that won't weaken your frame, or let water in. Go with U-bolts with a piece of flat metal strapping between the bolt ends, run it on top of the side boards so it won't rub your tubes.
You could also do like I do, and just tie them on. I use some flat webbing, get the pieces wet, tie a good TIGHT square not on the side towards the tube. Especially once a little dirt gets in the knots, they are not going anywhere. Good quality cord works to.
I use this option because there is nothing to cut a bare foot, U-bolts are brobly stronger.
Some people use one foot cam straps, which makes your side boards easily removable if you ever need to drop some weight.