Well... for the truly hands-on types out there, there is this
Looks like it would not be too difficult, but then you can get a "Shooting Chrony" for $80 on Amazon. It's the Red Box, but it's Green. The difference is that the Green unit lacks the splat guard and muzzle cradle. All Shooting Chrony's measure velocities from 30 fps to 7,000 fps at 99.5% accuracy (according to the company website).