Note that the floor corners are cut out to let light in. When swallows first look at the box, they need to know that the box is safe to use. It also lets rain water out. The roof has an overhang to prevent crows from pulling young out of the box.

Making the oval hole using 7/8" and 5/8" Forstner wood bits.

Nest boxes are best mounted as high as possible with a clear flight path. You can hose clamp nest boxes onto an existing steel pole.You can mount nest boxes on a long 2x2 and secure onto a fence post, then no ladder will be required.