This part uses four remote-controlled car (RC) shock-absorbers as spring loaded feet. The shocks are/should be adjustable so that they can account for the extra weight at the back of the printer. Otherwise, just find some the right size, and don't worry about getting expensive ones. I used some of these: https://smile.amazon.co.uk/gp/product/B07GPSL8HM/ref=ppx . In my case, the top of the shock just about squeezed into the hole in the top of the foot, which holds the shock in place nicely. There's room to put something through the hole in the end of the shock to hold it in place if necessary.

The foot has a little hole on the inside of it to fit a small grub screw to hold it in place on the printer. I found this wasn't really necessary as it seems to grip without it, but perhaps over time the PLA will stretch a little and it'll become useful.

The top and bottom need to be glued together (I used super-glue). Since the top is under some stress, I recommend letting the joint fully set before putting it into service ;-)

In terms of performance, I haven't made any careful measurements. What I can say though is that the table the printer is on shakes a lot less when printing a narrow section of infill (which tends to make the head move back and forth quickly over a short distance, causing a shaking effect). If you watch closely, you can see the springs absorbing that motion.

It's possible the bottoms of the shocks could benefit from a 'foot' (or weight spreader). In such a case, some felt or rubber on the bottom of the foot would provide grip and possibly some additional sound isolation.