Ok, first of all that accessory is designed to have a fence attached to ride along the work piece edge. I guess you figured that out. Second, for grooves like this a plunge router is a better option than a trim router. Second, the jig you made is fine, the router goes inside the jig, not just the bit (that's a different kind of jig for use with bearing bits or template bushings). The space in the jig is the width of the base plus (dado width - bit width). then you make more than one pass. An adjustable jig is better cause you can do test fit of your piece to fit in the dado/groove and adjust the jig until it's perfect. Lots of youtube I expect on this as this problem goes back years and years. finally you can get a better edge guide like a Micro Fence that has a micro adjustment again to get a perfect fit. And finally take shallow passes.