Much more info at the link, this was just to give people an idea...
My mind was wandering, thinking about where on earth I'm going to plant all the herbs I want too grow... <snip>
http://www.taunton.com/finegardening/pages/g00117.aspAnd here's some text...
"I found directions for making the planters in a rock-gardening book by H. Lincoln Foster, and then fine-tuned them into my own recipe. You'll need peat moss, perlite, and Portland cement. Make sure you buy straight Portland cement. You don't want any of the pre-mixed concrete or mortar you'll find on the market."
Nothing wrong with British Gypsom for this job, I guess it's a similar substance to 'Potland cemente', and I'm guessing that 'perlite' is stone or pea-shingle. To be honest I don't see why a straight forward concrete mix can't be used... I'll let you know how I get on with that, and if I need to bung in a shovel full of John Innnes number 2 I'll let you know that too.
Here's a pic or three...

"Planters require two stages of curing:
Curing refers to the drying of the material and to the chemical bonding that takes place. Curing has two stages: an early stage when the planter should not be moved, and a longer drying period following removal of the mold. The initial curing period takes 14 to 36 hours, while the latter takes about three weeks.
For the first 36 hours, cover your planter with plastic. The plastic makes it cure more slowly, and slower curing produces a strong planter.
After a day and a half, I test the hardness of the mixture with my fingernail. If I can scratch the surface, the mixture is still too soft and should be left for a few more hours and then retested. When it requires a screwdriver to scratch the surface, the planter is ready for the second stage of curing."
Sorry but the other link I have is no longer working - it was for making your own wooden planters... Maybe you know how to do that already!
Edit to add: I still haven't got round to making these... but to be fair it was a very busy year. This year's I might just fit it in!