UPDATE 20210324: Murata DHR series is EOL, Vishay 615R150GATD10 is ~ $5, and the Russian surplus has dried up.
SUGGESTED PART NUMBERS
Nominal specs from 545
In stock at Mouser
There is about 22mm vertical space between the baseplate and the tubes.
Box film caps fit just fine provided the three lower-voltage
parts are skinny so you can stack them.
Grid/Feedback/Cathode: 6.8nF 3kV, use 10nF 3kV box film, Wima MKP1W021005B00JSSD $0.89 6x15x27mm
If you allow 2kV, you can use Kemet F463DB103K2K0Z, R76UN2100SE30J or R76UI182050H4J,
or Epcos/TDK B32672L8682 or B32672L8822, or Nichicon PPB2201680KGL.
Tripler 1: 6.8nF 5kV, use 10nF 6kV box film, Wima FKP1Y021006F00KYSD $1.83 15x26x32mm
Or 10nF 6kV Z5U ceramic, Vishay 564R60GAS10, $5.86 ouch
Tripler 2/3: 470pF 10kV, use 1nF 15kV Z5U ceramic, Vishay/Roederstein HVCC153Y6P102MEAX $2.44 8x15mm
Between the two rows of ceramic strips except as noted.
On the left, place Grid flat with leads facing left.
On top of Grid, place Feedback with leads facing right.
Place the feedback resistor string to the right of Feedback.
On top of Feedback, place Cathode flat with leads facing right.
At the lower right, place T1 flat with leads facing left.
At the upper right, hang T3 vertically against the strips.
Below the lower right strips, hang T2 at an angle, head down/leads up,
avoiding the ground lug and straddling the filament wires.
Not a Component Engineer but I like playing one