I never knew it until a few months ago but it's actually feasable to get a locking "brake" handle from a bike shop. The button locks the handle back further than stock (meaning when it's locked you're not still like halfway in gear anymore) and it's got a 2-piece mount attached by 4 allen screws.
Not to mention the thing is like, rediculously solid and I only paid around $15. I think they call it like a parking brake.