Well, there's the hacky glue stick trick, or a pro product like Plasto wax, used in practical effects makeup to build scars and such, or disappear eyebrows!
The glue stick trick is frankly nasty, and I don't know how well it works over time. The glue stick is first applied against the grain of the brow hair, the hair is brushed back down, and the glue stick is reapplied with the grain of the hair. Let dry, and apply a second coat. Top with a concealer like Ben Nye super white, then coat with your foundation. No guarantees!
Plasto wax is easier, although you may have to order online. Just roll a bit between the fingers to warm it and work it into the brow. Smooth, and apply more as needed to blend the edges. Top with your foundation or concealer.
Brow powder and a stencil is typically used to build the desired eyebrow line real ones are gone.
I saw both methods used by makeup folks doing SFX head shots