I did the exact same thing with a half sheet of plywood. We had the plywood under the mattress of our old bed (sleep number baby now).
I have the plywood on 4 plastic sawhorses that I got for $15 a pair at home depot or lowes. I also used an old leaf for a table that I no longer have to use as the base for the machine. I also covered it with a padded vinyl tablecloth ($5 from walmart) so it is slick and I can't feel the plywood.
I have a LOT of room to sew now. but I must say, yours is SOOOOOOO much prettier than mine. I couldn't find a desk cheap enough...even at Salv. Army