Pallet carts are somewhat flexible and have a 3 point stance. I have done it before but a knee mill is working on the limit of one of these carts. For a cheap DIY try using 5 or 6 pieces of 2" pipe and roll the machine same as the Egyptions built the pyramids.
Your riggers are expensive - my charge out is $56/hour for machine moving and you wont pay for our lunch.
Forklift and or overhead crane is best for moving a knee mill, their center of gravity is relatively high.