Check these out... little pricey but i think the heater cores are all the same 4 screw pattern/size outlet flange, just different core sizes/outlet shape tubes.
https://boostdoc.com.au/collections/...r-core-outlets
Edit: tuzzio, was it the same guy that i bought the pcv/water bypass stuff from on the ka24 page? (Not nicetime racing) I mentioned it to him in DMs that making them in aluminum would be a good idea seeing he did a good job on the other parts.