I used 8 of these to make a box for my printer using good quality cardboard from packages. its designed for 5mm cardboard but ive included the step file too for easy modifications. The holes are 2.5mm and i used them to reinforce connection by passing 1.75mm filament through them to hold cardboard in place (works well)

layer thickness 0.32 mm, infill over 20%

Can be printed without supports on a well calibrated printer by changing over hang limit to 46 deg and placing it on the side edge (along the two  identical slots not base or top slot ,ive added the image of that print too).Use raft or brim. use support blockers on holes

For printing with support (what i preferred) place upright with single round holed slot on bottom. settings for easy support removal in cura: support type was zigzag or lines, 8% support density ,disable support brim, support x/y dis 1,min support area 2.5.they come off in a minute