Heat loss as pointed out earlier is worth thinking about. (1) The resistors
are designed to cool themselves, so mill the "heatsink fins" off. (2) If th=
e
block could be milled for a slot you could get more contact between the
resistor and the block. (3) It almost looks like you could bolt another two
resistors on; even if smaller, since every bit helps.

The other thought I had that wasn't covered already is that if you have a
cheap plentiful supply of resistors, you could subject them to large
overdrives and observe their failure modes.  (May be some good YouTube
material here :)  Then do successively lower power tests and watch how the
destruction time increases.  Maybe you can then predict how long they would
last at your desired power level.
--=20
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