Trying to figure out dimensions for unusual shape and size of my RMH

Hello everybody

I have Ianto's book, but my project has some special features,
because of where I build my RMH, and the shape I have to give it.
I have no clue about how to adapt some of the the dimensions to my project.
What is important and what can be changed...

My house is in a cave, so my whole house is already heating my home the way the RMH does!
60°F / 15°C inside is just not enough in our wet winter...
It is small, and I will not cook inside. The kitchen is close to it, sheltered under the cliff, but outside.

- The feed tube will be inside, with a glass door to look at the fire.
- The barrel will be in the kitchen.
(it will be made of steel and not with a barrel, so that I can make it higher)
- The heat storage will be inside.

-> Both the burn tunnel and the pipe have to go under a stone wall.

1) The burn tunnel will be longer, and I cannot double the size of the heat riser of course.

What is the maximum ratio between the length of the burn tunnel and the heat riser?

2) Then the pipe has to go back into the room...
The angle between the ash pit and the exhaust flue pipe will make a corner instead of going straight facing the hole where it comes out of the barrel.
To say it another way: the exhaust pipe will face the inspection hole.
Is it a problem or is it possible?
Will it slow down the flue too much?

If I put the ash pit under another side of the barrel, it would be between the barrel and the wall.
Then I would loose the access for cleaning by hand....

Thanks for any clue to help me go on!

Here is the kitchen viewed from the outside.
The RMH will be in the corner (well, just the barrel)
That is more or less in the middle of the photo, between the green poles.
The wall is wider than it seems, as the bricks are doubled with stone on the other side.

That is why I am interrogative about the length of the burn tunnel...

By the way, I forgot to say that this stove will not be for cooking, except may be keeping a pot warm...

I think I will not have to heat long, and my main motivation is to have less smoke than a regular stove, because I live under a cliff.

Will it work if my tunnel is about 3 feet?