To create an additional worksurface in my shop, I made an auxiliary top that fits over my router table. Simple construction using MDF and hardwood make building it a breeze.
I started by measuring the width of my router table. The depth is measured with the fence moved all the way back. (The fence could also be removed.) To size the top, I added 3⁄8" to the depth and 3⁄4" to the width to account for the rabbets in the three frame pieces. After assembly, simply slide it onto your router table.