Pennygale- do it!! It is so much fun. It does take some muscle mixing up the concrete, but it is well worth the effort.
My suggestion- try one in the ground without a mold. You will see if you enjoy making them without the cost and time of building a mold. (I use the 2 inch styrofoam insulation board for my molds. They last longer then cardboard boxes and easier to change the size then wooden boards.)
Dig a hole deep enough to end up with a trough at least 5 inched deep and as big around as you like 12-18 inches is a good size. If you want the outside of your trough to look like a geode do not smooth the hole, but leave divots in the sides. The bottom needs to be a bit level so the trough can rest on it when done. Line the hole with plastic and you are ready for your hypertufa mixture.
If you are impatient and accidently break one taking it out of the mold, just patch it back together with more hypertufa mix. I did this with one of mine and I think it is the neatest trough I have because of the added texture. I really need to get a picture of that one.
I have made and sold troughs for the past six? years, I just can't get enough of them!!
I started selling them because I thought

I had enough in my garden AND I wanted to help support my addiction. LOL!!
My heart starts pounding

just thinking about the tiny mosses and thymes creeping over the edges of the troughs.