I think the 2 box solution is acceptable.
jdragoset showed how he did it earlier in this thread and Gapsus used 2 boxes too in the thread I linked earlier:
https://www.eevblog.com/forum/repair/tektronix-465/msg672083/#msg672083
By using a plastic jiffy box and potting it you have options on where to place it ....within reason.
Thanks tautech, that is true. Sometimes I think I should stay away from trying to do everything as "original" and make it easy on myself. But the challenge is hard to resist
Certainly, physical sizes of parts have shrunk so much today. So, how did Tek make such a compact unit with all those components some 30+ or more years back, when there were no parts like today? Apart from the EHT tripler section, it contains a bunch of caps, diodes and resistors for the rest of the HV circuitry. I am amazed!
Another note: I did not have to use too many diodes to get to 13 kV. Only 4 diodes and 4 caps even though I hooked up 6 of each to begin with that gives about 18 kV. The schematic does not indicate what the EHT is even though it clearly says it is a x3 multiplier. I decided to stay at just 13 kV based on a 465 schematic where it indicates the anode voltage as 14-16 kV. Considering the age of this 465M, 13 kV should be a safe bet I think where I still have good intensity and focus.
Sadly, the replacement U 550 unit I got also has the same identical problem as the unit that was in the scope, which was hard to believe. It cost me more than US$ 100.00 by the time I had it in my hands. That was all down the drain to say the least.