brass will have better thermal conductivity then aluminum it's just more expensive and weighs more copper would be even better but cost more, have you thought about scrap yards? they some times have bar stock or atlest something that can be worked into something useful.
I dont know why people think aluminum is king on thermal conductivity but brass and more so copper is better it just costs more, 4 times as much if memory serves but if that's all you can get then i guess use it, otherwise scrap from audio amps have very nice aluminum heat sinks with usually one flat side to work with
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