Another vote for the little lamb boosters here too. They are really thin, fit in a teenyfit, and you can fold them in half to pop int he wetzone. We used them to boost nappies for nightime at the start
I used zorb in mine, I bought some and cut my own boosters to fit the Teenyfits, etc. and it worked great for making the nappy last longer but not adding any bulk. Still using them now in our small and OS nappies. I like the LL boosters as well, but the zorb is thinner and lasts just as long.