Removal is pretty simple, set the lower in a mill vise and use a 3/8 end mill to mill it out. Stay out of the Alum side wall. If you are dealing with the new style that has a block that is actually part of the lower find the true center of the lower bring your tool in and make a cut on each side to create the correct opening, be mindful of depth as well. In the past I have copied a
low shelf receiver to get the correct dementions. The material is soft and so a HSS end mill will work fine, how ever carbide makes the project a breeze. 20-30mins.