I agree the copper suggestion would draw in and spread out heat the best, but in the past I have found it difficult to tap. Is it easy to buy in bar form and I imagine fairly expensive? If it ever comes to immersing my electronics into transformer oil to keep it cool I'll take up another interest, that doesn't even rate as a last resort. I might consider a radiator and circulating liquid through tubes and a thermal exchanger with FETs on it but that would be my last resort.
Its no coincidence that the standoffs are all inline (~10mm high) I already pictured a long copper or aluminium bar from end to end, tap them in the centre for M5 or 6 and bolt the power inputs directly to the middle of the bus, lots of options.
If the HY4008's are happy in parallel without oscillations they can be bolted direct onto aluminium Bar I have lengths of 25 x 10mm xsection with one long and 2 shorter pieces. The long one is the + connection, the shorter ones are the choke connections again if tapped you could bolt directly to them. Then with strips of silicon pad bolt the Bars to a heatsink (with insulation for the bolts) and we have better thermal behaviour than individual pads on the FETs. All this fun still to be had......Edited by wiseguy 2019-05-18If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike