No there isn't one for the Adventure. Its seal would be compromised too much with a liner added. There's a simple way to get a similar effect though. Use a big barrel-style dry bag (say 120 litres) and roll the top open (instead of closed) to form a big soft bucket. Stuff it in through the hatch and then arrange it so that you have the opening right under the hatch opening. Works in much the same way.
By dry bag I do not mean your basic roll-top drybag (like the Hobie ones). I mean a duffle style bag, like this (but bigger): http://www.rapala.com.au/rapalahome/fin ... el_Bag.png
Side advantage of this is that if any water sneaks in through the front hatch, it ends up in the bag and helps keep the fish cooled off