: That helps. I did not realize that even if I supported my upper legs I would
: also need to support the above the knees too (was kind of thinking that
: the control would be with the upper legs, not the knees, hence I could let
: the knee caps go freely up towards the skin on the deck and thus have
: lower deck compared to if I had a brace on top of them).

If you're using a round cockpit with the two beam setup (masik and knee brace), the masik is basically just a support for the coaming and it doesn't have to be a tight fit on your legs. In fact, it may need to be somewhat loose in order for you to be able to get into and out of the boat.

: I am not sure yet how long I want the cockpit to be - I need to put together
: a "jig" to see what feels comfortable, I suppose. I've never
: paddled a boat with a cockpit of less than 28" of length, which due
: to my 36" inseam still meant I had to sit on the rear deck and
: sqeeze-in that key-hole cockpit legs-first.

I find that the VCP ocean size coaming fits perfectly with my 35" inseam. The front end of the masik sits just above my kneecaps. BTW, I find a thin, wide masik to be most comfortable. I use a steam bent beam that's 3" wide, 1" thick at the ends, tapering to 1/2" thick where my legs contact it. You can see this in my Webshots album on the subject at the link below.

: Once in, there was plenty of
: room vertically (12-14" from hull to bottom of coaming, depending on
: boat), where on the SOF I am tempted to experiment with a tighter fit -
: looks like 10" internal room at the upper leg and about 12 for the
: knees is the least I would be comfortable with due to my tight hamstrings

That's an individual preference that will probably change over time as you get used to the boat. You can always add padding to the masik later if you need to tighten the fit.

: (not to mention at least 9-10" for my size 15 feet)...

You'll need to angle your feet sharply forward (normal in an SOF) to increase foot room, so make sure that you don't put the footrest beam too close. Before you skin your boat, wrap the area around your feet with cloth or plastic wrap and test the fit. You may find that you need to adjust the position of the foot rest beam.