Welcome back Matt! And congratulations on getting your oven going.
Specifics of a design are beyond my skills, but if you built the oven according to plan, you’re already well on your way. Because you have a well insulated shell, you can use ordinary brick and mortar, with all the design choices that implies. You should have enough space around the oven to lay a course of bricks surrounding it, then just build up from there. Remember to stagger your rows as you did for the barrel lining in your oven, and the concrete blocks for your base.
You might be tempted to leave a brick or two out of the little walls you are building for ventilation purposes, but don’t. There will be space inside that will beckon to creatures wanting to winter over. Since you’re doing a metal roof, any latent heat will be conducted away by that.
For the roof, it wouldn’t hurt to include a heat-rated fiberglass break between the chimney pipe and the roof. Even better: if you kept the scraps from the blanket you used to insulate the oven, you’re home free. A flange around the pipe would also be a good idea to provide some weather sealing. Look to wood stove installation guides and suppliers for that.
Love how you’re thinking about this, and hope you’ll keep us posted along the way!