My nephew has knocked out a large new flame-cap kiln which we have yet to test drive. It was fabricated from 2 full sheets (1.6mm) to form the trough and another sheet for the ends. Dr Karl Frogner’s design cleverly limits steel wastage. The frame & ribs are 25mm SHS. The design for this 2000 liter kiln and other smaller sizes are available thanks to Dr Michael Shafer & WarmHeart Thailand but I suggest a donation to the orphanage they support in Northern Thailand if you plan to take advantage of their designs.
Basic materials for the kiln were about $400 and another $90 for the wheels & axle stubs. We got the plates bent for ~$140. Do get in touch if you want to get more details.