James...
Great tubs! They may not last forever, but they're cheap, and functional. You'll be root-pruning your trees after 3-5 years anyway, so that will be your chance to swap the old tubs for newer tubs, if they need to be replaced. If you drill drainage holes in the bottom, and the tree roots grow into the soil, make sure you get yourself a good truss before you try to lift the containers.
Alternatively, you could bury the containers half way... pile mulch up the sides, to cover the containers and keep them cooler. You'll be picking figs soon.
Have fun!
Frank