I use an inverter (600 W) to convert from DC 12 V to AC 220 V 50 Hz, but the wave output from the inverter is a modified sine wave, which causes problems when operating some electrical appliances (high temperature, noise, etc.) I also find it difficult to obtain a current inverter that produces a pure sine wave so I am working on converting the resulting wave from a modified sine wave into a pure sine wave using (a coil or resistor) and a capacitor, but the wasted energy when using the copper coil is as large as it is greater if you use a resistor.
The simplest solution is to rectify the output and wire it to a synthetic sine wave inverter.
I'm not actually kidding.
Given that this is a fair amount of circuit design (even with pre-baked (single IC) solutions available, a complete design still requires a handful of transistors, capacitors, several diodes, and a couple dozen resistors, and all must be placed and wired correctly on a PCB), perhaps you can begin to appreciate how bad a passive solution must be.
I can put it another way: if you had enough copper and iron for a passive filter network (which might still not even be compatible with the inverter as such; it might not start up into such a load, or other characteristics will be lacking as a tradeoff) -- why wouldn't you simply sell it for scrap and buy a better quality inverter?
More seriously: the most likely candidate would be a ferroresonant transformer. It's not obvious to me whether a modified sine inverter would be at all happy driving one of these, but suffice it to say, a 600VA unit weighs more than a few kg (hence, is literally worth a modest amount as scrap). They run hot, and are rarely used and therefore expensive to buy -- if you can find one in your market at all.