Okay, I'll give it a try. Conceptually the low pass filter can be the same as of the high pass filter, except the capacitors and inductors would be swapped. Your high-voltage generator may already look inductive, so by just adding a capacitor to ground you will likely get a second-order filter, but you might want to add another series inductor to avoid loading the high-pass filter.
Currently you are using a third order high pass filter, but you are likely not getting a third-order response. If your measurement equipment is high impedance, then the capacitor in series with the input is not doing much and can probably be removed or connected to ground.
Without resistors, you may find that the high-pass filter is highly under-damped, which would cause significant ringing. Adding some resistance to the filter might be in order. Something like the attached schematic might work (assumes output is high impedance). You might be able to avoid the resistors if the input and output look resistive. If there is significant DC current flowing, you might want to move the input resistor so that it in series with the input capacitor to avoid the voltage drop.
You might want to take a trip to the library of the nearest university with a electrical engineering department and take a look at
Handbook of Filter Synthesis by Anatol Zverev. That has wealth of information on how to build filters.
I also recommend you use SPICE to check the circuit and make sure it works as you expect before building it. If you do, you will want to come up with reasonable models of the input and output impedances to include in your simulator. The filters will interact with the input and output impedances, as well as with themselves, so the design is complicated and simulation is highly advised. If you don't have access to SPICE, you might look at
LTspice. It is free and has a strong following.
-Ken