Hi here,

Am a new user.

The disadvantage of a double-side printed circuit under the tracks is that you more insertion losses and what is forget here, is that the total capacity also increases.
Of course you should not mount this PCB flat on the metal but a little bit above it.

I have the best experience with "floating" filters with the coils is separate (with shielding).
If you don't do that, the harmonics will (happily) blow over.

Without special (high price) measuring equipment (Vector Network Analyzer) you actually have no idea what you are doing.

Marcel