I bought a small ballhead mount from Amazon for something like $10 that had 1/4" female threads in the bottom. Then I drilled holes in a few different positions on my thwarts and used a 1/4" thumbscrew to attach the ballhead. Simple, compact, and easy to take on/off.
My ballhead is relatively tall, probably about 4-5". It actually has two adjustments, the lower simply allows it to tilt back and forth and the upper adjustment is the ballhead. I never used the lower adjustment but the extra height it provided was helpful. Something taller would have been nice but it did better than I thought it might.
I'm using an Olympus TG-4 as well. I picked it up just before my trip. I've been pretty happy with it though after prolonged wetness it did pick up some moisture behind the protective screen in front of the lens. It lived in my pocket the entire trip so on rainy days it would be wet all day and then when it warmed up moisture would start to condense behind the screen. But it always seemed to dry out if I left it outside, particularly in the sun.
Alan