So I think I have partially solved this. It now seems to work but the cropping doesn't.
The directory structure was far from simple and based upon inspecting another plugin's deb I ended up doing the following:-
I placed:-
freeze-filter-0.3.3-linux/freeze-filter/bin/64bit/freeze-filter.so in /usr/lib/x86_64-linux-gnu/obs-plugins
freeze-filter-0.3.3-linux/freeze-filter/data/locale/en-US.ini in /usr/share/obs/obs-plugins/freeze-filter/locale
freeze-filter-0.3.3-linux/freeze-filter/data/effects/freeze_part.effect in /usr/share/obs/obs-plugins/obs-filters
That last may be incorrect which is why the cropping doesn't work, but the only other place I could find for .effect files was in /usr/share/obs/libobs/ which only seemed to have built-in effects.