I know that this is not a solution to your specific problem, however.... My DH found a used computer table (5 feet long) with a drop down for a keyboard for $15 at a college auction. He was able to stabilize the drop down part at the correct height and I will be using it for a sewing table as soon as we find time to rearrange the sewing room. I will be able to reach my bobbin to refill it from the front also where I could not if we cut out a hole in a desk.
Colleges often will have sales or auctions when they get rid of dorm furniture, dressers, desks, computer tables, etc. Keep an eye open and you can find some deals.