@hoyt
is there an 800V rated TO3?
see this thread:
Transistor isolation question
@FvM
i have not found the information you want - at least not all in one place
every article, ap note etc on mounting transistors, not just metal TO3, but plastic -220s -243s etc
do essentially the same thing - screws, nuts, washers and shoulder washers.
they do not talk about creepage distances or arc over.
neither does Horowitz and Hill (at least, i didn't find it)
@Zak
I doubt it is small,
and i doubt it is inexpensive
especially since very few things seem to be in TO3 cases anymore