Something else to consider, is that tanks can be repurposed, if they're clean, and you can support them properly (some tanks will require a 'cage' built around them to prevent collapsing/bulging).
Things like fuel transfer tanks, waste or clean water RV tanks, etc. are all possibilities, especially if you are comfortable with some plastic welding. But scratch building a tank will be a lot of work.
I use a 55 gallon open top (with clamping lid) drum for a buffer tank. It was cheap, like $30 or so on craigslist. I never tow with more than 15 or 20 gallons in it, though.