Thanks Doc, I hadn't forgotten!
Still waiting on some details, I'm getting a few favours so it's happening at its own organic, sustainable pace
At this stage I am looking at building the single batch model first. This will hold about 27L to the rim, and it will be around 300m wide / 600mm tall (rim-to-apex) before fittings.
For the lid I am planning on using a clear polycarbonate disc with a silicone seal, and a galvanised clamp band as used on oil drums. This should give a very reliable, air-tight seal with a minimum of fuss and low mfg costs. They should also handle a couple of PSI for those wanting to push with CO2. Stainless bands are available but around 100 times the price (no kidding). However. galvanised should be fine as it won't actually be in contact with the brew, and they can be replaced easily for a couple of dollars. The discs and seals will probably be custom parts, but I should be getting enough made to have spares available.
The larger size may be a bit more tricky, as I haven't been able to find galv clamp bands in appropriate sizes. I can get stainless ones in 400mm which is a good size, but they cost over $100 each from the only supplier I've been able to find.
I'm reluctant to go into an elaborate lid solution, as I think most of them are ugly, probably unreliable and labour intensive (and therefore time-consuming and expensive). Rather than jeapordise the project I'd prefer to build them to fit a standard plastic lid, which would suit most brewers, and those wanting something more elaborate can get it done themselves.
I am also planning to offer the option of a stainless coil welded around the outside. This way you can regulate temps quite simply by pumping something through the coil and insulating the thing well. It shouldn't cost too much to do.
I'll keep you posted with developments, and if anyone reckons they can help me get hold of drum clamps in different sizes please PM me!