Thick wall or solid part of an otherwise medium-fill part
I'd suggest creating a hollow tube with ~1 to 1.5&nbsp;mm walls. Then to increase the strength design your own internal structure rather than relying on the slicer to fill the part. This takes longer to model but you can design the strength of your part much better. 

Something like this 

If you were to then print this with 60&nbsp;% infill you'll have the strength and also use less material. If you happen to know specifically how your part would be loaded, you could then change the dimensions on that internal cross structure to take the loads better.