Sometimes, if you don't have any wood or tools to make a 'proper' form, or anything else like a bucket or a barrel, to hold your dry stone moongate arch up, you might just have to use a big rock. Here in Vancouver we used a huge slice of basalt column which was conveniently lying around available at Northwestern Landscape and Stone Supply during the workshop I taught there a while back.
I had been planning to take the chunk of basalt out, after the arch stones were built around it, but the garden feature looked so spectacular when it was done, with the form protruding both sides, we decided to keep it in the wall.
