High-voltage capacitors in space
Presumably this is a LEO application so radiation is not too bad- if it has to work in a vacuum that would rule out normal electrolytic caps, as you say, which would tend to be the most compact in that range. 

You might want to consider film caps such as the PP MKP1848C61050JK2 from Vishay. It's not small at 18 x 28 x 32mm but meets the spec, can perhaps fit in your space (assuming something like a cubesat) and is probably more reliable than MLCC caps (at least that's my experience).

Without a ripple and and regulation and max current spec it's hard to speculate what you could get away with, but I'd be tempted to look at a capacitance multiplier (perhaps with a Zener) post-filter with a substantially smaller filter cap. A base-emitter diode should be added to basic circuit.







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