A few photos to show you how the mould-making is going. I finished the clay accordian form and here I'm building the surrounds, ready to pour the plaster. There's clay on the outside if the boards to stop leaks, and also between the form and the boards on the inside so the edges will be neat. That's the worst part in my opinion, as it's so fiddly to make it really smooth.
Here the form is fully surrounded. You'll notice I stuffed up slightly - I didn't put the boards together properly, so the last "wall" was too small for my boards. Never mind, I just used clay. In retrospect that might have worked better for all four sides because it was much easier to push the wall onto the form and make a neat join. I have seen clay walls collapse mid-pour....resulting in rapidly setting plaster all over the floor...so I usually go for the boards. I'll have to give this more thought if I end up doing more of these.
And the mould fresh out of the surrounds. You can see the end where the clay wall was - it's a bit wonky and has sharp edges, but they can be smoothed out. You can probably also see how untidy the sides are - that's where I used extra clay to join onto the boards. Tomorrow I'll do some cleaning up and take another picture. Hopefully it'll be a lot smoother and ready to go!