How can I block nearly every site for short periods of time to improve my focus on work?

The best way at present may be to script a temporary block of all other sites.

If you want to block all sites except for example.com and work.com, and both example.com and work.com are already allowed in your configuration, the following is one way to add a 90 minute focus session.

on Windows:

cd Desktop
pluck find allow | findstr /v when | findstr /v example.com | findstr /v work.com | pluck replace allow "when now+90m block" >a.txt
pluck import a.txt
pluck + when now+90m block text/

on macOS or Linux:

cd Desktop
pluck find allow | grep -v when | grep -v example.com | grep -v work.com | pluck replace allow "when now+90m block" >a.txt
pluck import a.txt
pluck + when now+90m block text/

If you’d like a ready-made script, see focus.sh.

TODO: write a similar script for Windows.