I made one out of a '67 Atala road-racing bike. Back then, road racers used long wheelbases and not so tight angles, making the frame perfect for the job. There is also plenty of space at the rear triangle for a fairly wide tire.

I put a Guerciotti cyclocross fork on it and attached the rear brake points myself, using Mafac brakes. I also attached brake cable bosses to the frame, since everything was clamped on back in the sixties.

Finally, I have a few sets of off-road tubular tires with wheels to match. But I've only raced that bike a few times, and that was a very long time ago. I'm not even sure if my off-road tires still hold air.