alan stewart : I hope that I can be a little help, and maybe marshal my own thoughts. Here I am paraphrasing what Ernie And Erica Wisner Say, and I am pushing
my luck, if I don't get it right Erica will assume I was sleeping in
class 
The 1st part is easy, build it outdoors first when the Barrel goes on you put up a Vertical
Chimney, this allows you to prove to yourself you have built a functioning Rocket, and then you can lay out the horizontal runs, add the vertical chimney back on
and test again !
Generally when you have your base finished you want to fire it up right away to promote drying, if you cob in the bottom of your barrel to the burner base and allow
the cob to set up around the Hot and Expanded metal rim you will lessen the chances of cracks appearing later from letting the base cure/dry completely and then
firing it !
Here I want you to Find the Forum
Thread ''
Rocket Mass Heater Shippable Core in the
rocket stoves forum and clickon-> it
Then I want you to scroll down and look at the third picture, in this example any horizontal hole can be made through an arc of about 300 Degrees, add in the 360
degrees the rocket burner base can set, and your choices become almost infinite!
It is only important in this case that the wall of material of the Rocket burner base be thick
enough to support the horizontal pipe (and clean out 'T' and Cap), but leave
enough room for the Transitional Area to be a couple of times bigger than the Constant cross sectional areas of the
Feed Tube, Burn Tunnel, and Heat Riser!
A round shape holds the greatest volume in the smallest space, your Rocket Burner Base can be any shape that fits your space remembering the need for most of the
Feed Tube and Burn Tunnel to be insulated outside of the bricks making up the Tunnel!
I actually do not have a clue about a lid strap. If one could get barrels in good condition for $5 ,and get two 1/2 barrels, this might seem a cheaper way to build a Cob
Thermal Mass Bench, but I use mostly scrounged light weight ductwork for my Thermal Mass, so I have not investigated its structure Sorry, perhaps some one else
will attempt to
answer that part of your question !
All sections of ducting stove pipe
should be secured to its neighbors with short sheet metal screws or pop rivets this,and a thick sturdy wall that can be re-enforced
with hardware cloth or
chicken wire. This should hold everything rigid during the Cob sealing of the Pipe before laying in stones, rocks or other heavy dense materials!
What is your location, 8'' is generous for a small cabin ! Though you did not state so, I am assuming that you plan on Refractory brick for your Feed Tube and Burn
tunnel. for the good of the craft ! Big AL !