Rocket Mass Heater - top of barrel shape

I want to use a water heater tank as the barrel in a RMH. Any I have seen have used the top end which is domed upwards. However, The top has at least 5 holes and two tabs but the bottom has only two holes.... I would like therefore to use the bottom as the top, but the bottom on top would look more like a permanent wok. (time for pansit and stir fry)

My thoughts so far are:
1) domed would take less room
2) domed directs the gases gently around the corner (good or bad?)
3) I have only one, so I can't try both ways
4) the tank is 4ft high so I would have some extra material to plug holes

Any ideas?