get some garage door cable or aircraft cable and a few cable clamps and turnbuckles and a couple of garage door pulley wheels. Hang the cable between trees or poles with a downhill angle and hang a target underneath the pulley wheels. You can rig up a release with a small steel clamp similar to a clothespin and a long string or light twine or rope. You can change the speed by changing the downhill angle of the cable. If you are sharp with the mechanics of things you could even rig a return pulley from your shooting station with a long twine or similar. You could also rig an automatic latch to catch your target when you get it returned to the high end.
Good luck.

Bugleward