Yeah, recessed bottom with holes connected into the channels.
I have read what BillA suggested, think I'm gonna try 1.5-2 mm base in one chunk 10 mm thick, and I have some 5 mm thick 5 cm wide copper here for the flat top.
I don't want to sound dissapointed, it is just that I thought that I made my first setup 'from the hip', and considered it somewhat crappy and ghetto, but had some nice temps with crappy block. Guess I expected more to happen on the temp side, not to idle on 2 C higher...
Yup, live and learn, let's make some blocks!
g.l.: if you need some 5 mm copper, I can send you some. I have one chunk 50 cm long. Wanna share?