Actually, this is not that hard to do. You need to find a piece of perspex that has the same size as the router bit you want to use. Then attach this perspex to the router plate using double-sided tape or screws. This may not be the prettiest solution, but it does the work. There are also better solutions out there. For example, the Porter-Cable router plate that has a template system which allows you to change the height of the router bit just by changing the template. Or the Bosch T-profile router plate that has a height-adjustable fence.