For $9.99, I bought a single lot of 16 snubber boards for the same Toshiba IGBTs as the ones that I own. They came from the same seller as the one who sold me the four IGBTs on heatsink for $50.
There are two kinds of boards in the lot, eight of each kind.
One (marked with 1 on the photos) seems to be a simple RCD design and features WIMA capacitors, Ixys FRED diodes, etc. It seems to be missing the resistor between J11 and J12 (see pictures).
Another (marked 2 on the photo) seems to be some sort of RCD design with an inductor for some reason, and actually with resistors.
I am thinking that I should simply install these snubber boards, one on each of the four IGBTs, and not think too much. To that, I would add an extra big RC snubber just in case.
I am leaning towards #2.