What might be easier is to get a tabletop double the size of your current desk (or more), attach two legs to the far side, and use the current desk as a support for the front half. So the new top is resting on top of your current desk, and is perfectly flat with no seam. You might find a wooden poker table at a thrift store, or a small breakfast table that you could take two legs off of. If it's slightly bigger, like a couple of inches all around, it wouldn't matter.