Make my own support material?

I want to print an object that will need some support material. On some smaller objects I've let slicer do it's thing and it would change the way it would do support material from object to object - even though they were copies of each other. Some parts worked out well and the support material popped out easily, others took some hacking and digging and a few broken xacto knife blades. The part I want to print needs some support for some larger areas and it seems to be doing that fine but it also wants to put some pretty solid support material in some tight spots like slots for 3mm nuts that would be a pain to dig out. I'm wondering if I should draw in my own instead of letting slicer do it for me, and if there are any guidelines for doing it. FWIW working with ABS.

Thanks

I've done that sometimes, using a single extrusion width wall which can be removed easily with a knife. Oftentimes slic3r gets it wrong, some support pilars fall over, some support impossible to remove etc. You can probably do it more intelligently and selectively, saving print time and filament in the process.