Hi kitui, welcome to our little corner of the 'net.
For something like this, you would certainly need a bearing guided straight bit.. either top or bottom mounted bearing, both would accomplish this.
If it was me, I would make a template the exact size of the cutout that you need. Once you have the cutout made, place it where you need the opening made. Draw the outline on your work piece. Then, cut most of it out with your bandsaw or jigsaw. Don't cut it all with your router.. the bit will over heat and fail. Cut to within 1/4" to 1/8" of your line. Then, fasten the template in place with double-sided tape. Then, with your bearing riding on your template, cut the remainder out with your router. This will leave you with rounded inside corners. You will have to square them up with a chisel, or, if you like the way the corners look, leave them. The choice is yours to make.
Search "template routing" on youtube for lots of examples of using a template.
Sometimes I wonder whether the world is being run by smart people who are putting us on
or by imbeciles who really mean it.
Last edited by BrianS; 01-23-2018 at 11:16 AM.