currently, your 3 choices for geographic filtering are:
- place (can include multiple places) – ex. https://www.inaturalist.org/observations?place_id=18,19
- rectangle – ex. https://www.inaturalist.org/observations?nelat=35%3D&nelng=-90&place_id=any&swlat=25&swlng=-100
- circle – ex. https://www.inaturalist.org/observations?lat=32&lng=-95&radius=500
as a variation on filtering by place, you can also exclude places (ex. https://www.inaturalist.org/observations?place_id=18¬_in_place=110679 or https://www.inaturalist.org/observations?lat=32&lng=-95&radius=500¬_in_place=110679).
others have requested the ability for more “freeform” geographic selection (ex. https://forum.inaturalist.org/t/ideas-for-a-revamped-explore-observations-search-page/8439/3) or other related functionality (ex. https://forum.inaturalist.org/t/ability-to-search-observations-near-within-range-of-a-place/14986).
that said, based on the use case you’re describing, it sounds like filtering by a place may get you what you need. you would just need to create an appropriate place, if it doesn’t already exist. (if you have questions on how to create a place, feel free to ask. or if you search the forum, i’m sure that topic has been covered in excruciating detail many times.)