Greetings!
There are 2 ways (that I know of anyways) to achieve this, one which works in Cura 2.3.1 and one which does not (but does in a later version (I am using Cura 3.4.0)), so depending on if you are able to update Cura or not one or both might work.
Option 1: (not available in Cura 2.3.1, but perhaps easier)
You can make it slice so that if a part of a layer has less than a predefined area it will skip infill all together (and instead print it solid):
For this:
(note that it is hidden by default, so select "show all settings" in order to find it, And you might need to slice a few times to get a number which results in desired results)
Option 2: (Available in Cura 2.3.1, but more effort)
Changing slicing settings where models overlap.
(For this you might want to disable "auto drop models to buildplate", or get creative with models)
Step 1) Make/have a second model which is only the parts which require different slicing settings.
Step 2) Open this (together with the original model) in Cura.
Step 3) Align/position the models correctly.
Step 4) Print Setup: Custom.
Step 5) Select the "mask" model (ie: the model which is partial, and will be used to partially affect the original)
Step 6) ToolBar -> "Per Model Settings" -> "Select settings" -> "Infill Density" -> "close"
Step 7) Edit values according to your desires.
I hope this was useful, or was able to help in any way, if you have questions, please do ask them :)
(And I know the second might be slightly ehm... Not beautiful, but it works. If anybody knows a better way, please do tell :) )
Greetings,
~Sjaak