Yes I’ve done it many years ago on building sites (I wasn’t the only one).
Well it wasn’t exactly a workbench it was just something you knocked up to fix the upside circular saw to – a table shaped object.
It works surprisingly well, the slot for the blade is made by simply plunging the circular saw, and the guide is just a length of timber screwed down.
And the switch has to be jammed on, so make sure you can quickly unplug should things go wrong.
Obviously it works best for repetitive rips as the guide can’t easily be moved. And not having any guard or being able to quickly switch off is obviously a little dangerous.
I would suggest screwing the circular saw to the bench even if you have to drill holes into the base as I wouldn’t trust clamps not to suddenly release without warning due to prolonged exposure to vibrations.
Better still buy the right tools/kit.