Although having a sled that is slightly wider than it is high, in your case 1/4", may not seem like a huge deal, the Maslow is capable of higher precision, and this error may be a signal of larger errors elsewhere. There are several threads addressing this issue which make for interesting reading…
The first step is to learn about and run the Calibration Test Pattern…
This exercise will indicate just how accurate your machine really is, and give you the language you need to work with others on the forum to correct your problems. In my case, there were several issues, but the biggest gains were made when I rebuilt my sled using the following procedure…
Getting the spindle on the router perfectly positioned at the bottom of the triangle formed by the chains is really important, and worth the extra effort. Assuming you are using the ring, triangulaton, and a 1/4" bit, an easy way to see if it is centered is to turn on the router, plunge the bit 1/10 inch, and rotate the sled on the ring. If you still have a 1/4" hole, you are dead on.
Another issue for me is that my chains are not the same length… the right one is about 8mm shorter than the left. Although there has been some work on compensatiing for this problem in the software, I am not sure that this effort has reached prime time. I recently purchased 50’ of chain, and hope that getting both my chains from the same production run will solve this issue.
Meanwhile, might I suggest that you have some fun with what you have. A lot of cool things are being made with these machines, and I am certain that many of these projects were made with systems that are not perfect. For instance, a lot of neat signs and shapes are being made, and small errors really do not show on those things.