Alpacka has a clever way to get a boat to fit: wedge your feet into the narrow bow space and push your back against the back tube.
However I am a Yak size by that method, yet I prefer a Llama’s stability and high volume. So I stuff a 3" thick semicircle of minifoam (http://www.nrsweb.com/shop/product.asp?pfid=2076) into the bow on the boat’s bottom at my feet. It can be less than a semicircle or more to space you well. It is light and keeps your feet from hitting the bottom hard and from going to sleep. It does not come out when the boat is flipped as it fits very snugly, but comes out with a tug or boat deflation.
Mine fits perfectly in the bottom of my HMG packs when hiking and in my boat keeps my feet from getting scrunched.
In the back, I do not rely on the air tubes. I have moved my seat forward and use this Jackson Kayak backband (http://www.kayakshed.com/jackson-kayaks/surelock-backband-kit) clipped to my metal hip d-rings with locking carabiners. Luc Mehl showed it to me, so I bought one for each boat.
With the foam pad and the backrest I do not need thigh-straps up to and including Class IV–.
With the foam and the backrest I can get the exact fit I want inside a big high volume boat. Adding thigh straps locks me in for paddling at the limits of my ability and rolling.