Dome height is entirely up to you - it really depends on your oven's use.
For pizzas, you'll want something lower that can reflect the heat onto the top of your pizzas to melt your toppings.
For bread, you'll want something higher, that will diffuse the heat more.
A 42" pizza oven is generally 10"-16"high -12 being considered very aggressive (difficult to build and pretty much only for pizza).
Bread ovens can be as high as you wish - you may want to stack bread on racks. For bread, the key point is not to make it too big - you'll want to have enough dough in there to create a humid atmosphere when cooking. A dry atmosphere will lead to pale crusts, while a nice humid one (from lots of dough in there) will lead to a golden crust. This is my issue with my oven - if I bake a whole batch, I'll have too much bread... if I bake what I can eat, its pale
The key thing is the door height with relation to the internal dome heigh. I can't remember the exact ratio, but I think it was 66% or something like that... it'll be on the fb guide...
If you take a look at this post you'll see how I made my indispensable tool from some thin studding and a hinge. The top end was an L shaped piece of metal I had lying around in he shed. The only other advice you need is that if you use an L shaped holder on the end, then the studding must line up with the centre of the brick, otherwise you'll get steps between courses.