I think you're doubling the appropriate range for longitude (i.e. [-360,360] instead of [-180,180])

I was indeed. Thanks, and fixed!

Cf. also http://extremelearning.com.au/how-to-generate-uniformly-random-points-on-n-spheres-and-n-balls/

Marsaglia (1972) is available online, http://doi.org/10.1214/aoms/1177692644