However, I didn't want to permanently alter my sawhorses too much because I use them for all sorts of things.
What I came up with is simple, quite sturdy, and can be taken down and set up very quickly.
2 sawhorses that I already had
4 simple 2x4 joist hangars (upc 0081942100607, I got mine at Menards).
small diameter wood dowel (i used some 1/4" that I had)
pair of 2x4s of matching length (I used a pair of 4 foot and a pair of 6 foot ones for 2 sizes of work table)
sheet plywood of a size to go over your new table frame (I used a 3'x7' piece of 3/8" plywood that my neighbor was throwing out)
Hammer (to drive the nails)
Hacksaw (to cut off the nails and notch the dowels)
Power drill and bits (for the dowel holes in joist hangars and cross members)
Pliers (to bend the cut nails)
Metal file (to take the rough edges off the cut nails and the holes drilled in the joist hangars)