After we removed the original water tank (see here), we debated on whether to make another rigid tank or install a flexible one. We read the pros and cons of each and finally decided that the flexi tank was way cheaper, easier to install, and we'd only be missing out on maybe 10 gallons of holding room. At $100 per tank it's easy enough to just replace it in a few years if it ends up leaking. The main things we were told to do for prepping and to make the tank last longer were to make sure the area was free of anything sharp, lay down a cushion for the tank to rest on and not chaff, and make sure it's fastened down and not able to roll around. We used a closed cell material that is typically used under floors. It was all we could find and was cheap (btw, this stuff works great over the port lights to block out the sun and heat)! We just threw some contact cement on it and put it everywhere to be sure the tank would be nice and comfy.
Since our tank is in the keel, we knew we wouldn't have to really fasten it down because it can't go anywhere and is very snug. We bought a rectangular 40 gallon tank from Island Water World for about $100. It's a bit of a funny fit and has to twist a little to make the most of the keel shape. I think the max we've gotten is about 32 gallons so far because it's not exactly the right shape for the spot, but that works for us! We also have a 30 gallon triangular flexible tank under the v-berth so we can hold about 60-65 gallons.
These aren't the greatest pictures, but you get the idea!