I took the legs off of my plexiglass table so it would set flush with the sewing table and it works just fine.

Another idea for smoothing out any potential rough edges - you could cover the table top with white contact paper! Just leave it long and wrap it down around the hole. I have a wooden table to the left of my machine and it was an Ikea cheapie unfinished wood...I was too lazy to finish it with varnish so I covered the top of it with white contact paper and it's great! No way any splinters are going to catch me or my fabric, and also makes it blend in with the white melamine table that's behind my machine. Quilts glide right over it. I had some cords that had to run across the tables and I used the white contact paper to cover those up too - you can see a lump, of course, but my quilt slides right over them and they don't go anywhere. I love contact paper!! :)

When DH did mine we called Dream World Northwest in Eugene, Oregon and they told us the dimensions to make the hole and then we could buy the plexglass insert from them. There are also some directions in a blog at DIY Sewing Table Plans Blue Dinosaurs. Hope this helps.