With RC paper in sheets, the paper lies very flat.

So if you would like a borderless print, just place your paper on the baseboard in a particular spot, and allow the enlarged image to spill out just past the edge of the paper. Using a smallish aperture is a good idea as well (for the depth of focus).

You can use cardboard "L"s and tape, or a larger easel with the blades adjusted to something like 10.12" x 8.12" to create the "nest" for the paper to sit in.

You may have to trim the result slightly, given RC paper's tendency to wear at the corners.