I used the standard issue Shapeoko endmill (#201 .25" square endmill). I follow the chart here for feeds and speeds:
So from that for HDPE, I set the following conditions for my Toolpaths in Fusion 360:
Feed: 80 in/min
Plunge: 40 in/min
While the chart says DOC is .125", I went less than that. I was concerned that it might be a bit aggressive, and I didn't mind for this project if extra passes were required since I wouldn't have that many passes or a long milling time anyway. I did half that value when I planned my stepdown increments in my toolpaths. Oddly enough, I missed changing it for the 2D contour cut on the outside of the shapes (I input the .125 value before having second thoughts) so when that toolpath ran, I had a bit of a heart stopping moment when the tool plunged in much further than I though it would. However, it did not cause any problems, so maybe that .125" value is really verified. It did however remind me that I need to order a dust boot soon, as it created a pretty impressive shower of plastic chips that flew about 6 ft or more in any direction.
I will say that I have been reading here how some folks go conservative with their feeds and speeds in certain materials, but I think these were good settings for HDPE. I immediately checked the bit after milling was done and I shut it off, and there was barely any heat in the bit. There were a few chips that had spiraled themselves onto the upper part of the mill, but they were not melted.
I let Fusion 360 set the tabs, and while they held, I would change the distance between tabs to be 1.5" vice 2" which would have given me two tabs on each side of the clamps instead of one. I would also increase the height of those tabs to .1". I think they were set at the default of .06" or so, and they held together but barely.
The material was 1/2" HDPE measuring 6" x 12" (although I modeled only in a 6 x 6 box). Ordered it from McMaster Carr: