I just put a rock on top of Page Down.

I wonder whether there's a market for "key weights" that fit a key exactly and are heavy enough to keep it pressed. Depending on the occasion I have used heavy implements (not exactly rocks), a variety of wedge-like tools like scissors and other things to keep a key pressed, but a dedicated tool such as a properly-shaped paperweight might have been much nicer ;)

In linux you could write a script with xdotool to select the window you want and spam whatever key/sequence of keys you would like.