QUICK DESCRIPTION: Don't bother with the first three files, because they turn out incredibly fragile. They're just there to document my learning process. Print v3 if you have an older printer which struggles with overhangs, or v4 if you have a newer printer and are feeling a little more adventurous. NEW: v5 goes back to the v3 style, but has the flat surface of v4 to make attaching the magnet easier (cylindrical cavity for the magnet, rather than a conic cavity)
These are 16 magnetic push-pins I made for the 10x3mm magnets I got bulk from Amazon. Should print fine without supports, not sure about the need for a raft. Build surface required is only 66x66x20mm, so you can duplicate the object if you want multiples of 16 to be printed at once, or pare them down if you want fewer. I also made them 6mm apart, but this may not be the best choice. I wasn't sure how far apart they needed to be for best printing (less stringing). Good luck!
The reason for the odd and primitive (wink, wink!) design of the pushpins is to avoid difficult-to-print internal overhangs and bridging, to save on material, and to be pretty quick to print.
It should be trivial to scale this design to the size of your magnets. You might want to give a 10% or so fudge factor if you're scaling down (e.g., if you have 5mm magnets, scale the object down to 55%*, rather than 50%).
*Why 55%, and not 60? You're scaling up by 10 percent after you've scaled down. So if you want to make the object 25 percent smaller, let's say, then it's:
current size * (1-percent to scale down by) * (1+fudge factor)
So for a 25% scaling down, that's 10mm * (1-25%) * (1+10%),
which works out to: 10 * (1-0.25) * (1+0.10)
which becomes: 10 * 0.75 * 1.10
Working out the last multiplication first (which is totally permissible under the commutative rule), it becomes 10mm * 0.825 (82.5%, meaning you've scaled it down by 100-82.5 or 17.5%)
This yields 10 * 0.825 (82.5%) = 8.25 mm
UPDATE: I noticed that the conic surfaces are kind of bumpy, and realized that I had forgotten to max out the number of facets in Tinkercad. So the v2 file is the same, except that all of the surfaces are now much smoother, and the conic shapes aren't noticably faceted (64 faces vs. 15 or so).
UPDATE 2: "Magnetic Pushpin (10mm, single).stl" is so you can arrange the pushpins however you like, for example, arranging them far enough apart to be printed one after the other with Cura's "Print one at a time" setting.
UPDATE 3: I finally got to print this file, and the handle portion broke off immediately. I have now posted Magnetic Pushpin v3, which has a 3mm thick handle (instead of 1mm thick), and should be sturdier.
UPDATE 4: v4 is a totally new design which makes it much easier to install and hold the magnet -- but much harder to print. Haven't tried printing it yet. It should print without supports on a decently high-quality printer using at least 3 walls and 0.1mm or better layer height.
UPDAT 5: v5 goes back to the v3 style of pushpin, because v4 was just too fragile, although it looked more like a real pushpin. Like v1-3, print v5 at about a 101% scale to have a little bit of room for the magnet to actually fit properly (this is easier to do in a slicer than it is in tinkercad). So, v5 is a little more challenging than v3 to print, because of the internal overhang, but is much sturdier than v4.