Well, if you want a smaller pre-modded heater core there's the
d-tek jr-120.
I've never seen one, other than on a web page. Their (now discontinued?) 6x6 cores were good.
Then there's the made-for-pc-cooling radiators, BlackIce and ?thermalchill? (may have the latter name wrong).
I've used a number of BlackIce radiators with good luck (used black ice
pro because I was cooling for silence not overclock and was looking for minimal air pressure required for OK cooling)...
Bob