This Small 1000 (21 Spline)  to 540 PTO Adapter is the perfect match for your Shop Dog. It can be conveniently carried in the Shop Dog's hitchpin and PTO adapter carriers.
 
This is also a terrific PTO adapter for tractor use. This adapter reduces the tractor's PTO from 1-3/8" x 21 spline to 1-3/8 6 spline (540) and extends the PTO shaft 4-1/8". It has an overall length of 6-3/8" and is rated for tractors up to 70 HP.