I was going to use a clamshell for mine with mineral wool insulation. But I didn’t do it, because I wasn’t sure how to keep it dry if it rained.
The hornito used a 55 gallon barrel where they cut it and added 1/3rd of another barrel (lengthwise) to it then put a new top on it which gave it like a 2" gap around the barrel.
You have the lift so it might be a lot easier for you to slice and dice a few barrels then weld them. Then you use the lift to drop it right over the char barrel. You could put insulation on the inside (provided you left enough space ie you might have to use a half a barrel), but it may not be needed. that way you aren’t losing heat if you only run 1 barrel. I think you will lose more hot heat if you try to share the heat between barrels, unless you have another purpose for the large clamshell.