Kettle drums are round, bronze, and pitched.
The resonators in a marimba are usually made from aluminium, the length of tubing required for the bass notes exceeds the height of the instrument. Some manufacturers, compensate for this by bending the ends of the tubes. Others, expand the tubes into large box-shaped bottoms, resulting in the necessary amount of resonating space without having to extend the tubes. This result is achieved by widening the resonators into an oval shape, with the lowest ones reaching nearly a foot in width, and doubling the tube up inside the lowest resonators.
I know a welder, who runs his own welding company who can put us in contact with manufacturers if necessary. For the time being we should all go to the metal scrap yard in Shep-bush near me. We can go to my sisters house for some tea afterwards. Anyway, we need some beaters and some elastic bands to test resonance. With the help of a glue gun, we can make beaters with sticks and bouncey balls and felt cover that can be removed. I have all these things at home, all these things are in uni shoppe too.
After onedotzero Im going, come along! It might seem a little early to look for materials but I disagree with that because the only way to figure out how to do this is through trials.