[JFB]

a piece of metal to prevent the bolt from opening, held in place with a magnet?

the slide stop can be replaced with one that will lock closed instead of lock open

 

[COBrien]

What about buying one of the BX-1-1 mags from Ruger? Some say they're pointless, but it won't accept more than 1 round, and will be cheaper than any "drop-in" conversion.

The JG Bolt Lock that JFB references would work, but they're apparently some $70. See a review HERE. You can contact the manufacturer at the link at the bottom of that page.