I practice with 2 Strats. Most electric guitars weren't designed for playing in a sitting position, but I always felt that Strats were more comfortable than average. Les Pauls in particular always slide right off my leg when I play them in shops. But since I started practicing these long hours, I've been less satisfied with old faithful.
There are 2 conventional ways to hold a guitar when seated: over the right leg or over the left leg. I switch between them frequently, and I like to prop up my foot with some books or a footstool, a couple inches on the right or up to a foot on the left.
Unfortunately, neither position works for me; I have to twist my torso one way or the other. I did some investigating in the mirror with my guitar strapped high and my arms in comfortable, more or less symmetric playing positions. If I sit down in that position, the wide part of the guitar body falls right on top of my right leg. That's exactly in between the two playing positions, and totally impractical. Junk.
Until I achieve one of these guitars (notice the bottom contour on every body), I'll have to stick with switching positions so I don't tweak my back.