Topology for ideal cutband response (-20dB per decade per order) for passive, low pass, constant output impedance filter
The only way to get a higher rolloff is to insert additional poles.

Shown below:


Vout (green) your filter
Vout1 (blue) additional pole with 40dB rolloff
Vout2 (red) your filter with parasitics (ESR and ESL on cap and ESR and capacitance on inductor)




You'll probably need to get X2Y caps an parallel them with normal caps to get the rolloff to continue at high frequencies.