Years ago, I bought two inexpensive (faux wood) cabinets at Lowes. Put them with backs facing, the width apart that you want the knee space to be. (That will depend on the amount of space you have, and the top that you use.) Also at Lowes I bought a piece of wood the size I needed. The cabinets are 24" wide and 11-1/2" deep. I spaced them so that the farthest edges are 48" apart - so the wood I needed for the top is 24" by 48". DH rigged up the shelf under the top to hold the sewing machine using L brackets, and cut out the size needed for my acrylic insert. It's not beautiful, but it works great and gives me some storage as well. I've probably had it for 20 years.