You can make block feet using any size form you need and attach them to your planter/trough with a little hypertufa or mortar mix. You can also form them at the same time you make your trough. Put a piece of Styrofoam in the bottom of the cardboard box and either cut out the corners of the piece of Styrofoam or use 3 pieces of Styrofoam to form a plus sign. Use a plastic dry cleaning bag to line the box and cover the Styrofoam. When you place the hypertufa mix, fill the feet cavities first and tamp the mix in the feet to remove any air pockets. Fill the rest of the cardboard box to make your trough.

Thank you so much! You explained this very well.

Thanks for the tip! We are going to experiment next week and every tip helps.
Tally HO!

Just watch the plastic bag. If you tamp or pack too much it can embed itself in the mix and make a weak spot. We've got parades and picnics this weekend or I'd be playing in the mud too. Have fun