You are not the first person to come up with this solution which is a very practical idea. When the time comes to replace your router most of the 2-1/4 HP "combo kits" have through the table height adjustment on the fixed base. A simple swap from this base into the plunge base lets you free hand with ease. Be sure and post photos of your boxes in the Show and Tell forum. Include information such as wood type and any special techniques you have developed.