Perhaps you need to cascade two or three lowpass filters.
because commonly the stopband of lowpass filter can not be far away, often not bigger than 2times the passband edge(because the step impedances are not infinite high or low).
to extend your stopband you can use defected ground structure or use suspended stripline techniques.
In simple, as Amicloud told, the DGS structure can offer simple size of LPF, for I think the ground have to be etched about 10mm(for 5mm of a and b, it comes around 3GHz cut off frequecy). I believe, the ordinary stripline with 400mil length and 300 width with one open circuited stub of 400 mil length can give such LPF, for instead of DGS.. Amicloud!!