How to design your own support structures for FDM printing
See below for a method to manually model support structures.
Create a surface that is offset in Z from the feature that requires support by one or two layer heights. There are many ways in which to do this, but I typically create a sketch profile below the supported region, inset from any walls by 1mm and outset from the top edges of the supported region by 1-3mm.
Then extrude that sketch profile to the supported surface to create a new body/component.
Next, move the support body down in Z by one or two layer heights (two layer heights, 0.4mm in this case).
Using the bottom of the object that you want to print as the slicing tool, remove the same gap distance from the bottom of the support structure to make it flush with the bottom of the object requiring support.
Shell the support structure out to an even integer multiplier of your nozzle width to minimize impact on print time. I used 1.6mm in this case.
I either chamfer or fillet the inner corners to give the structure more surface area to hold onto the build platform.
If supporting a large span, you can add ribs that will bridge and help support the proper compiling of the support structure.
Finally, it's always good to double check the toolpath.