I have this cheap workbench pictured below. As you can see I tore out the tool tray because if I fill this in and add some on to the side I can make it 24" wide which I would like. I just ran a router sled over the top to flatten it. Since it is a cheap bench I do not want to put much money into it so my question is this. If I can find some relative knot free common 2 x 4s, fir or pine, why not run these over the jointer and saw to width and glue them to the side? Are these likely to bow and make the top uneven or will they be OK?