Multisite frontend site selection

Hi,
Great plugin. Works very good.

I have a question about multisite search. I want to give my visitors the ability to select which sites they want to search in my network.

For example
I have added three sites in the multisite options tab. These sites are:
– Finler Encyclopedie
– Finler Opleidingen
– Finler Shop

Standard functionality is that a search query always give the results from all three sites. But what if a visitor wants to exclude 1 or 2 sites from the results. See screenshot to see what I mean.

I can’t find a solution. Is it possible? And how could I implement this?
Kind Regards,
Scholto