Slight change of plan.
I cracked open a failed (but genuine) RJK5010 MOSFET (20A, 500V) and it has a die size of 6x6mm = 36mm
2.
Assuming that 20A/36mm
2 = 0.55(5)A/mm
2 then the IGBTs i had would be 12.25mm
2 * 0.55(5)A/mm
2 = 6.8A (yikes).
6.8A is a far cry from the advertised 20A so i will not even try to plug these back in but rather i'll go the MOSFET route.
BTW i knew the IGBTs were not genuine but i didn't think they were THIS bad (i expected something like 10A).
My Toshiba MOSFETS, on the other hand, are genuine, i know because i pulled them from a LG plasma TV.
They're 2SK2698 (500V, 15A, 0.35Ω), i had four but used one to replace the blown RJK5010 mentioned above (and fixed a plasma TV

).
So i'll have only one per side but that's not a problem as long as my gate drive is correct.
I also checked the switching transistors from the ATX power supplies i took apart and they're like 10 or 12A rated so in theory using them would have been better than the fake IGBT's!
I actually might make another welder just to test these transistors out, mini spot welder for battery tabs perhaps?