The type of furniture would dictate how you go about converting. You will want a door and visibility. This article might help you out.
Donna: I usually don't post interest things... but these are great (do-able) ideas!
Here's a cool idea: https://www.youtube.com/watch?v=9fxcsyS8RSc
Here is a cool one from an old crib (a little involved though... ) https://www.myrepurposedlife.com/repurposed-crib-dog-crate/
Here's a fun one: https://www.youtube.com/watch?v=MwrZx76p26c
This is a simple idea. Instead of building it you could use an old table. https://www.hometalk.com/16743626/beautify-your-dog-s-crate-with-this-simple-table-build
There are some cute ideas here: https://www.rover.com/blog/cool-dog-crate-hacks/
You could use a coffee table up side down and add a top and make sides