Here's what my DH did to modify an old desk. He removed the drawer and mechanism. Being a carpenter, he was able to stabilize the machine shelf using metal L brackets (I think that's what they're called). Here's some pics that might help.
Then we had a friend of ours make an acrylic top that fits my machine bed and the opening around it. It's not pretty but it's functional and so much cheaper than the actual bed inserts.
I have a higher-end machine and it sews quite fast (not that I use it that way often) and have no problems with vibration.
Hope this helps.