I took k1nc41d's pick holder design and removed the rubber band guides, because I don't like rubber bands, and added holders for a spring I sprung from a clicky-pen. The opening the picks come out of is large enough for a thick weight pick, or two mediums, but I haven't had an issue with pulling just one out.
As the original was licensed GPL, this is too.
You'll print the three pieces, either together in one go, or seperate and then clean them up so the pick-looking piece slides easily in the box-looking piece and the lid piece sits on the box-looking piece. When you put the pick piece in the box piece, you want the peg on the pick piece to be aimed towards the peg on the lid piece. These pegs will hold a spring in place.
Take it all apart again, and find a retractable pen with a nice clicky spring. Destroy the pen. Get the spring from wherever it landed under the table.
Depending on how long it is, you'll have to snip it shorter, so it fits in the box piece between the lid and pick pieces. You want the spring to be just long enough that it holds the pick piece at the top of the box piece with a little force when you put the lid piece on. You want the spring to hold the pick piece against the top (open end) of the box piece without the spring falling out of place, but not pressing so hard that you can't get picks in.
You'll need some glue. Did I mention that? Well, you will. Something that will bond well with the plastic you are using. Acetone could work. So could a drop of gorilla superglue, which is what I'm using.
Use a drop of glue to hold the spring on the peg of the pick piece. Once that's dry, use another drop of glue to hold the other end of the spring to the peg on the lid piece. Let this glue dry too.
Now that it is all dry, slide the pick piece into the box piece and then use a drop or three of your glue to hold the lid piece in place on the box piece. Hold it until the spring won't push them apart again.
Now let it dry over night. Tomorrow you can put picks in it. Don't put picks in it tonight, the added pressure from the picks on the spring could push apart glue that hasn't set yet. Now put your picks in! And then take one out and ROCK ON!