TBH, I don't layer much....
If it's cold then I wear my drysuit with either my Unions Suit (IR) or their Thickskin layers (just 1 top and 1 bottom).
Other than that, I wear neoprene pogies, capeline (patagonia) mid-weight socks, Nike ACG's, and a skull cap.
I'm comfortable, in that setup, down below 20 deg.
Hope that helps. I hate feeling bulky when on the river. You have to work too hard to do anything.