A spring-mounted ball-isolator designed to fit onto the legs of the Printrbot Jr and reduce the vibrations transferred to the table/floor/shelf etc it is sitting on (and thus the noise produced by the printer) .
Based on the design by lazzymonk, I adapted the "base" cup and the spring base into one piece (i.e. no glue required) and adapted the "top" cup to fit onto the legs of the Printrbot Jr.
There are two types of "cup" in the download - three "normal" ones which fit onto three of the legs of the Printrbot Jr and one "different" one which has an extra slot to house a nearby protruding screw which holds down the electronics inside the body.
I modelled this in Inventor to make it easier to tweak the parameters between edits (sorry freeware tweakers!). This version should be just right - a nice, snug fit around the Printrbot Jr feet and a strong (but not stiff) spring action in the base.
Each cup is 3mm deep (so you should use ball bearing > 6mm diameter) and 25mm wide. I opted for an 8mm ball bearing as I was able to buy grade 10 (low grade number = really smooth) ball bearings relatively cheaply from Simply Bearings (UK: http://simplybearings.co.uk/shop/index.html).
Smaller diameter balls will bring the two cups closer together (more chance of them clashing or sticking together) and larger diameters will not give the ball much room to move around within the cup (reducing the damping and risking the ball rolling out of the cup).
You could use a marble or even print a ball bearing (definitely needs hi-res!).
At a push, you can fit 4x bases and caps on a Printrbot Jr stock printbed. I used a smaller skirt (2mm from the model as opposed to my normal 5mm) which helped.