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.
If your already buying closed cell foam for your feet why not buy a little more foam and just make your own backband, Far lighter than the Jackson backband, just as stable and quite a bit cheaper.
Here are a few picture links of a early version I made last year out of a piece of scrap foam. I have since made one out of thicker foam but the thin foam works just as good.
This is not the strap I use but you get the Idea, any strap with some tri-glides will work. I use a loopie strap and a small plastic quick release 1" buckle. Just glue (cheap contact cement) the foam pieces top and bottom and leave a place to run the straps. http://dl.dropbox.com/u/57909144/Kirk/IMGP3591.JPG
All you need is foam, a saw/knife or scissors, 1/8 inch bungee cord (4 contact points 2 up 2 down to keep it suspended), contact cement and a loopie strap with a plastic quick release buckle… I have about $10.00 invested instead of $49.95 and saved close to a pound in weight.
I have been using the backband and foot block for most of my trips now that are anything Class III+ and under and leave the thigh straps off as thigh straps make my old creaky hip joints ache.
With a seat moved forward and back back on the back rest attached to the thigh strap D-rings and feet pushed on the mini-cell blockI feel as snug a fit as with thigh straps. With thigh straps I feel like I am locked in (but of course I exit easily, too).
The handfull of people I know who have been using the Jackson backband have made it as permanent a part of their kit as thigh straps. And all those who have tried the foot block keep using it as the foot space in Alpackas is a bit tight.
Alaska Kayak Academy keeps Jackson back bands in stock.
Also have the foot block that Roman shared as well.
I agree with Roman, since I added these two items to my Yak, I don’t use thigh straps in water under class IV. Being locked in from foot to hip makes a huge difference