My first response was "on top of" -
especially when I want to use one of them.
(That's because I had three of them under a table/desk for a while waiting to get repaired)
I've found that it's more comfortable for me to set a portable machine on what used to be known as a "typewriter table"
It's two to three inches lower/shorter than desk height and it's easier for me to control the fabric when the work surface is
"lower."