This is a sifter/filter I made to be able to change the hole size according to what you need sifted. I used it to separate aquarium gravel/smaller stones from larger stones.

To customise the sizing of the holes (requires Blender, or export to something else):

-open filter.blend (in Blender) -select the mesh area/center object -scale the mesh object until you have the hole size you want (it will only show what intersects with the object 'Circle' on the 'Intersection' collection, according to the boolean modifier on the mesh).

Notes: -filter_applied.blend is the same as filter, but with modifiers applied (ie. ready to be processed if necessary, and you can't scale the filter/change hole size in filter_applied.blend) -filter.stl is the blender stl output, run through the Netfab stl fixer tool, here: https://service.netfabb.com/ (I don't know a better way to fix stl's that slicer will like).

-If you modify the mesh, be sure it lines up with the bottom of the outer ring. Currently both objects have their origins set to the lower-most z-vertex, so both should just read '0' on the Z position. This is so the printer doesn't try to mess up the bottom by adding strange/small supports in a grid pattern that will surely become a mess.

Printing: You should try to really push the z-spacing of your print head as close as possible for the first layer, so that when it is printing the first layers of the grid pattern, it doesn't mess any up. This happened to me a couple times, but then I fixed it by: -ensuring the STL's bottom layer is all the same z. -ensuring a very close first-layer z-height. -setting (PLA) filament extrusion multiplier to 1.01, at temperature 215. -setting heatbed temp to 70.

Hope it's useful!