PAR Electronics makes some fine Omni antennas. I've used the OA-50 in my attic with very good results. I've also used stacked OA-144s, OA-222s and OA-432s with good results when roving. All three of these are small and may not even be identified as amateur antenna if placed outside and not out in the wide open spaces.
73 es GL,
Dave N8AG