Easiest material to build the bell for 6¨inch BBR

What would be the easiest material to build the bell for a 6 inch system of a Batch Box Rocket.
What can i use to the cover the top of the bell?

My take on it.

http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

Easiest would be loam bricks, those pressed mud, unfired bricks also known as adobe or raw bricks sometimes. Although in the higher region of the bell, namely everything above the riser's end, should be shielded by one or the other kind of heat resistant material. Like firebrick, superwool blanket or the like.

Second easiest would be just fired brick with mud mortar. Or build it out of firebrick on edge with normal brick or adobe on the outside. It all boils down what materials are available and affordable to you. Even hollow earthenware bricks filled with mortar or mud could serve as the main building block.

what about rammed earth with a metal plate on top of it (cobbed in) ?

A metal plate on top will get awfully hot and will expand too much to keep airtight. Rammed earth is also a good way to build it but a bell out of adobe bricks would be easier to shape, in my view.

In order to close the top, you could opt for shaping the bell as a beehive or adding small pillars inside in order to be able to close the top with bricks. Or a combination of those two, of course.