As you can all see from the photo, I've already made the LPF. In time I will make this available as a kit, I'm only held back temporarily by sourcing connectors, I have another device needing them that will make up the rest of an order.

It's a 7-pole Chebyshev design. Circuit diagram of LC network below.

The component values were chosen as the closest preferred values to those derived from a QUCS simulation.

The inductors are wirewound ones, I suspect multilayer parts would not have enough Q.

And it all fits with 1206 SMD parts neatly on a 15mmx20mm board with a 10-way socket on the underside to mate with the first 10 Raspberry Pi pins.

The connector is an edge-launched SMA - it's quite small, and not impossible to solder by hand.

I think this is one of the smallest Raspberry Pi add-on boards made, if not the smallest.