New progress:
Picked up a 1970 Chevy Blazer heatercore which is the same as what pHaestus has been using (it has decent performance and it fits well with two 120mm fans):
Also, I picked up a Comair Rotron 24VDC fan and two 120mm 12VDC fans all for $20. Hopefully they will work well. In the background, you can see a JBL speaker cover. It just happens to be the right size to fit over the Comair Rotron fan. We'll see if it impedes airflow a lot or if the Comair even works well. If it doesn't, I've always got 120mm fans to fall back on.