Built it and took a fullscreen screenshot with GIMP to figure out the width/height/x/y coordinates I wanted and tested with Google Meet. Working perfectly!
$ sudo apt install slop
$ slop
<selects an area on screen>
1719x1403+1080+277$ clipscreen $(slop -F "%x %y %w %h")
NB. The lack of quotes around $() enables wordsplitting to occur.
$ clipscreen $(slop | tr -s "+x" " ")