I actually did get around to plugging the holes directly under the power resistors on those heatsinks, even though we all know it makes absolutely no difference whatsoever.
Start with some 5/16-inch aluminum rod, face and center-drill one end, turn down about two inches to a scant 0.200 inch diameter, saw off a stub on the end.
Grab the stub in the 3-jaw Sherline chuck clamped to the mill table and slice off 0.240 inch slugs using a teeny slitting saw and manual CNC.
No pix of the setup, for reasons that made sense at the time, but that project gives you the general idea.
Repeat three times to get 3 x 6 = 18 slugs, plus a few spares.
Clean out the heatsink holes, clamp a bar (covered with tape) on the flat side, butter up the holes & slugs with JB Weld epoxy, squish ’em in place, scrape off the excess epoxy after a few hours.
Actually, this was a thinly veiled excuse to get my shop assistant some Quality Shop Time on the lathe and CNC mill. Wouldn’t you do the same?