Rx100 LCD Hood
There IS provision in the Menu for setting the camera LCD to "Sunny Weather" which is supposed to increase brightness considerably (well it does do that as I know..) and you are then supposed to be able to SEE the LCD better in such brighter conditions. Frankly I find that I just don't like viewing that way but it's worth a try...
Otherwise I know of nothing specific for the camera...there ARE already well-established items like the Hoodman viewer..or even the Delkin which were popular a few years ago..similar sizes to suit the RX100..might just be worth trying. I used a Delkin on an older Fuji (F31 era) ..they flip up/down on a kind of hinge arrangement..certainly may well be some sort of answer , albeit adding just a bit of depth to the camera (it sticks onto the LCD edges a bit like the GGS type Protectors stick on...)
A Delkin on Amazon UK...
|Wood Panel Truck, Retired by Buzz Lightyear|
from Random Items - Challenge 22
|The Falls by jdaniels10|
from Show us LIQUIDS!
|Silence by JP Zanotti|
from Blown highlights