ok. so you’re trying to make a place or places for a project.
i think the best way to do this is to combine all your sampling cells into one multipolygon feature. (note that it should be 1 feature, not 300 features.) so you’ll get a place that contains all your sampling cells together. how to do this technically depends upon how your cells are defined currently. are they individually defined in some sort of GIS format already? what sort of GIS program(s) do you use? (if you use QGIS and your grid is defined with each cell being its own feature, you can open the advanced editing toolbar, edit your grid layer, select the cells / features you want to combine, and then select “Merge selected features” rom the advanced editing toolbar.)
the benefit of creating a single place that contains all your sampling cells is that there’s much less to manage in iNaturalist. the downside is that you won’t be able to query directly in iNaturalist for a specific cell.
i assume that attilalengyel is working in Hungary, and that country is not so large nor has so many observations that adding a place here should cause any issues. a multipolygon consisting of up to 300 polygons should not be overly complicated either, assuming the cells themselves are not overly complicated. (i’m assuming that a simple grid should mean the cells have relatively simple geometry.)