Support for internal corners

[Stabby]

Hi everybody
I am quite a newbye of 3d printing and much more newbye of Simplify3d software so I'm having some problem in supporting the internal corners of my objects

For example in the following simple part I would like to use a low infill (10-15%) but I am sure the the indicated corner will collapse while printing
https://img.bi/#/oXh55Fd!T0VxowZ3C0eweJ ... LXALhQiOfS

In fact if I go to the preview in that corner the printing will not have any support
https://img.bi/#/WJ32DMY!oxjzAAHeWoRg19 ... zKIf1AEtPN

How can I manage this without using a really high infill?

Thank you in advance

[JoeJ]

Can you attach a factory file (File > Save Factory File)?

Using enough top solid layers will probably fix the problem regardless, but it's always useful to take a look at your exact setup