First, make sure your makeshift tool is built well enough. I used trailer spring shackle plates from Tractor Supply to provide clamping force on the bottom of the lower control arm. 5/8" threaded rod through the shock holes, doubled nuts with fat washers for safety.

When reinstalling, I used a couple of cheapy mini bottle jacks along with the threaded rod to put the arm into place. This would not have been necessary if I had an engine in the car. Without an engine, it's too light to use the weight of the car to help compress the spring.