think of it as a tiered cake where the tiers are:
> narrow rectangles (take a 12x18 and cut into 3 - 12x6 strips)
> and all the tiers are the same size
two such cakes cut and stacked would give you three 4"tiers of two layers each.
for a cake that would stand about 12 inches high.
for a squarish tombstone that is 12x12x6.
could cap it off by baking 2 - 12" rounds and splitting them in half and using 3 of the halves standing on edge at the very top of the cake to give a rounded top. that'd make the final cake about 18" tall by 12" wide by 6" deep. this size cake would feed at least 130+ people.
if need less, start with smaller sheet or even a square.
example: if start with an 8x8 square -- cut in half you get 8widex4deep.
stack up 6 layers (three 8x8 2"layers split) and now have 12" tall x8wx4d and then cap with one 8" round split in half for a final height 16". Down to around 60 servings
of course the whole thing would have to be doweled and boarded just like any multi-tiered assembly.