I am thinking of using aquarium gravel . I stripped down a gas water heater for my RMH , the tank bottom has an upward curve , in this area i want to cut the flue at the top of that curve and fill in around the bottom and come level , that way i figured i could put a piece of metal on top to build on for the new brick flue . Does this make sense or should i reuse the 4" round flue and attach it to a 4" square fire box and feed like most people do . Thank you .
It would be easier to respond if you could include a picture or diagram.
In general, I only like 4" diameter systems for cookstoves or very low-mass heaters; they have too much drag to push through much horizontal mass. But a number of other people have built 4" heaters they like, so if you are following on someone else's success then don't mind me.
I also like to see the firebox completely lined with masonry or refractory materials, since clean fire is way too hot for most kinds of metal to survive without warping or burning out over time.