Add a locating pin to index your shovel (or whatever) and keep it from moving.
http://mandamus.typepad.com/onefullplate/2010/10/quick-fist.html
http://mandamus.typepad.com/onefullplate/2010/10/quick-fist.html
i can understand a locating pin, but i would be leery of drilling through the handle. maybe 1/2" at most, but even then i'd try to drill into the metal part of the shovel (of course if you get one of these, you can drill anywhere - http://www.russopower.com/products/king-of-spade-products-all-steel-spade-ks-d/)
if you're going to drill through the handle, you may as well just mount it with that bolt and a wingnut. durr