I’ve worked on my transient generator some. It can now generate a 240V swing in 20 nano seconds (the green trace below). The yellow trace is the output protected by a clamping diode. It was able to tame it down to 26V. That is not quite good enough (I’d prefer that be below 20V) but I’ve got new Transzorbs coming in the mail which will hopefully take a bit more of the edge off that peak. Assuming that peak is generated by an inductive kick because of a low side disconnection (which it was in this case), that 26V represents a only a 13V spike across the MOSFET which is actually just fine and well within spec. So, while I am not satisfied, the transient protection does appear adequate.
I’d like to get a some better test gear but until I do, my home made stuff will have to do.