I think fit is a very important thing in getting your roll. I struggled for two season in a boat that was too loose. First, I didn't think it would be that big of a difference. Second, I was afraid that I wouldn't be able to get out of the boat if I needed to do so. I'm now to the extreme of a tight fit in my boats. People who are shorter than me sometimes can't get in my boats. I think it has improved my overall boating as well.
I cannot fathom my knees falling out. It's impossible. I do give up some comfort, having a ratchet backband and releasing it during the calmer sections lets the blood back down to my feet.
Fit takes time. I have spent hours cutting foam, taping it in, trying it out, changing it around and finally glueing it in, repeat. My recomendation is to spend some quality time in your living room with your boat over the winter. Maybe bring some extra foam to the pool and experiment. It's amazing how much extra room there is in the feet when you move from being on solid ground to being in the water. I've had to add two or three inches from the living room process to the on water process. Also, try building up where your knees hit with foam. I'm 5'8" and have to put in a few layers, you'd likely need more.
Quick fixes - I'm not a fan as I like the stability of foam, but the Jackson Happy Feet have infinite adjustability on the fly. Just learn how to use them so you don't feel like your feet are balancing on a balloon. Something else I used is the Happy Seat. It's not something you put under your bottom, but something that supports your legs and really gets them up in the thigh braces. I've gotten away from using it as I've worked on the fit of my boat. If you have worries about getting out, it may be a little claustrophobic (you can get out, try in a pool first). Both would be things to try.
Hope that helps if you are in the EZG or a different boat.