This is very difficult to do. Some software (Simplify3D for example) can allow you to do different setting for different layers, but I don't know of any that could do different settings for different features on the same layer. The only way I know to do this is to have two models of each feature and print them on top of each other.
Depending on the layer height, I would honestly just print the whole model at 100% infill. The side walls are 2mm thick, so at 0.5mm extrusion width and 2 perimeters it is 100% solid no matter what the infill setting is. The only negative is that it will take a longer to print the base/top. This is the way I print these.