I've done the inductor and capacitor wave form shaping before on cheap inverters. The actual capacitance I found worked satisfactory is around one or two uf per 100 watts(standard motor run caps). With a inductor sized for the right current capacity. A fair sized toroid (about 1.5 - 2 inch dia) with around 20 -30 turns of wire just heavy enough to carry the inverters maximum output capacity seems to work with the least amount of trouble.
Its not a great sine wave but its closer than that brick it tosses out!
It cleans up that inverter hash you get on TV's and monitors and helps variable speed power tools work more efficiently.
However if the inductance or capacitance gets to high some cheapo inverters get glitchy and wont work right or at all!
If your running sensitive and high dollar electrical gear, why are you trying to get by with a crappy cheap ass inverter? Its just asking for trouble!