Laser-Cut Stencil.
Here's a popular wall-side or roof-side painted signage common on old barns.
This stencil packet includes two configurations - paint through the stencil for just the lettering onto your structure surface, or first paint a solid frame background and then the lettering onto that background. (see examples images below)
Stencil is laser-cut into heavy manila paper, so it should hold up well for multiple uses. Use a "painter's tape" to mask around stencil onto your structure surface. Get a tight fit up against your structure surface and use either a "stippling" technique with a brush or a spray paint technique from a distance straight onto the stencil. Try a practice piece to test your technique.
O scale stencil lettering box measures approximately 3.5" wide x 2.0" high. (see other scales for other sizes of this same stencil.)
Signage without background:
Signage with block background:
