Hello everyone, just wondering if anyone has tried fitting a loupe to the flip out on the 300?

I have not. A lot of comments on the LS-300 mention concern about the durability of the LCD mount and some have reported it loosening with use. I have not had any issues with mine, but the only thing I attach to it is a Hoodman hood which is very light. I do use a Zacuto EVF which works very effectively.

Sorry, no I have not. Like Luke Miller said I have a hoodman but that is it. Sorry I could not be of help.