Fox James

If you can buy vermiculite board then most of your problems will be solved.
So much depends on the materials available .  

The wooden box full of stones will not be a very effective method at all, certainly not compared to solid mass but I guess it might work for you as a compromise.
A far more up to date and effective method is a bell or enclosure built around the stove, in theory this might be an option for you.
So a basic description would be replacing the more traditional barrel with a solid mass around the stove.
An even simpler method might be a steel box ( barrel alternative) with stacked bricks around three sides.
Basically a piped mass system ( where a pipe is covered in clay) is well out of date as bell systems have proven to be more effective and easier to operate..
Is there a reason why steel barrels are not readily available?

The J tube is the only, easy to build, reliable rocket stove, but you still need to address some facts like the internal riser really needs to be insulated with some form of high temperature insulation.
The whole J tube can be made from fire bricks or even clay bricks, in an ideal world lightweight insulating bricks would be perfect as the stove could just be built and taken apart very easily.
I doubt in anybody outside of the environment can really understand the situation or imagine exactly what is available?
The basic priceable of how a rocket stove works is based on a very high heat situation inside the stove and that needs to be insulated. Perlite, Vermiculite, rock wool or ceramic fibre?

Eric, you might like this one too?

Oh yes there are very visual hybrid stoves, I have specialised in building visual stove for quite a few years!
Please look on my youtube channel for lots of ideas that actually work from simple J tubes with amazing vortex displays and lots of glass to the extremely visual vortex stove with hot plate.

I can look out of my window and see four different  varieties of Eucalyptus and I live as far away from Australia as it is possible to be, but I believe there are over 600 different forms.
As an avid didgeridoo builder I can confirm that the species has an amazing range of different textures and hardness depending on the species! In any case I would not worry about burning any of them in a rocket stove.  

andrew stevens wrote:any suggestions what sort of insulation / material i should consider for insulating the water tank ?

Rock wool would be ok but like most high temp insulation it needs to hold lots of air so you cant really tightly strap it in place.
In an ideal world a second skin 2”+ away and filled with loose vermiculite would be very effective.

These type of ideas are often presented on the forum but rarely do we see the results.
I really dont know how well your idea would work for you, J tubes are really great stoves, the internal riser makes the concept very user friendly.
However a 6” J tube built to the common spec will struggle to maintain 300c on the top of the barrel even a 8” wont run much hotter apart from peak burn when they can reach 500+ but maintaining that sort of heat is not so easy.
I have made several J tubes that differ slightly from the standard spec and do get a bit hotter but you still need a good strong draw and smallish dry wood to maintain 500c .
The first gen batch box stoves get much hotter but do have a tall riser.
My point is … a gas burner flame runs at 2500c so 300-500c on top of the barrel might not meet your exportations?  

Hi Tom, I often wonder about what box stoves are available in the States?
Though out the UK, France and much of europe we have a huge choice of highly developed stoves, triple burn, pre heated secondary air, catalytic and non catalytic to name a few. Most have an insulated fire box and air wash viewing window, they are all low emission and mostly smokeless.
To be honest where I live most box stoves are used for visual appearance rather than heating, wood is an expensive fuel and all new homes would only need a few candles to heat a room due to the ridiculous amount of insulation and airtight rooms!
However box stoves remain very popular and we have a very good store displaying all the latest stoves.

It says on the box that it is made from an Alkaline Earth Silicate (AES) fiber I dont know exactly what that is  but, it has been much discussed on other forums and it is said to sinter at a lower temperature that ceramic fibre so in fact may be slightly more dangerous?