Slice it with two walls - the thicknes is 4*0.4mm exactly, so it can be printed without drill-infill vibration. Make sure the walls connect firmly. It helps to over-extrude slightly.
If you print the second copy mirrored, you can start testing with the back feet.
It takes some force to assemble. The inner piece can be clicked down to the botton. Do this after you have forced it in place (the photo shows how to slide it in).
The springs are very stiff. The Ultimaker has all its weight at the top. If the springs are too weak it can build up oscillation. Watch out for this effect at the top, not at the springs.
After a few months, my back feet were notably bent. This was expected: when you put permanent load on PLA, it will creep over time. (ABS too, I'm told.) The design can handle creep to some extent. After 2.5 months, my back feet were lower but still lifting the weight (see photo above). The material I used was the PLA/PHA mix from colorfabb. I don't know if PHA makes creep better or worse. Anyway, creep deformations are permanent. You probably will have to switch front and back legs after half a year, and eventually reprint.