Design for a 6" firebox out of 1.25" × 4.5" × 9" firebrick

It looks like my only local supplier of firebrick carries them in these dimensions - 1.25" × 4.5" × 9".  I wonder if anyone has a design for a firebox for a 6" system out of these?  I'm not seeing a nice way to put them together without a lot of cutting and waste.

This will go beneath a rocket oven w/ a rigid board insulation heat riser so I just need the feed tube and burn tunnel leading to a ~6×6 (somewhat flexible on exact dimensions here) opening to connect to the riser.

With a 6" system having a CSA of about 28sq", you could get away with a burn tunnel that is 5.75" tall and 5" wide. Normally the bricks making up the burn tunnel are offset so you can make it 5" wide and 24" long (assuming a 16" feed and 48" riser for standard 1-1.5-3 ratio). If these bricks are standard dense firebrick (not the light insulated ones) then the thinner size can be helpful as you'd normally want some insulation surrounding the burn tunnel like perlite, so less heat is absorbed in the tunnel vs reaching the bell/stratification chamber.

Since the whole setup is usually covered with cob or some other finish, the internals don't matter as much. Edit: unless you're planning a minimal size burn tunnel, without much mass/insulation? I've only ever seen a burn tunnel for a rocket oven made using dense brick splits like yours with insulation fiberboard around it, with a metal outer shell to protect it. So if that's the plan then definitely I'd cut the splits to exact length to avoid any protrusions.

As a square shape is  said to have about the same flow resistance as a round shape of the same diameter, I think you could make the burn tunnel cross section 6" wide x 5 3/4" high using whole splits. The first course of the sides lies flat (1 1/4") and the second course is vertical (4 1/2"). 6" width plus 2 x 1 1/4" = 8 1/2", so there would be almost no overhang of the 9" wide tunnel roof.