Hi Lisa,

This is what one of our staff recommends:

People have made troughs that way, but it’s used more for a free-form trough with more rounded/organic sides than a traditional mold can accommodate. It sounds like you’re making a fairly large trough, so there will be no easy way to lift it out of the ground, if your intention is to display it above ground once its cured. It’s going to be very heavy if it’s large. The easiest thing then is to build a plywood frame. You will need an outer mold to hold the outside in place and an inner mold to hold the inner side of the wall.  The materials are lightweight compared to real stone, but it is still heavy. The easiest thing to do is to build a large mold like shown in the blog post. Alternatively, you could build up the outer portion of your mold with soil or sand if you want a more organic shape. The trick with this is the trough can only be so deep as there is only an outer mold. 

Alternatively, the North American Rock Garden society handbooks on troughs could be a very useful resource. You should be able to get this through inter library loan. https://www.nargs.org/nargs-book-service