The older, brass B&L Cinephor Series II lenses are often pretty inexpensive on the bay. I have a 5.75" that covers 4x5 with some extreme swirls, and a 6.75" that covers with less nausea-inducing effects.
The barrels are stepped, so it's easy enough to cut a tight fitting hole in a wooden lens board and press the lenses into place.
That being said, I had issues with the wooden lens boards cracking with changes in humidity, so I sketched up some lens boards with a longer 'skirt' to cover more of the 'step' in the barrel and 3D printed them with an opaque filament & high infill.
The example image was taken prior to flocking the inside of the lens, which makes a pretty substantial difference in contrast by reducing reflections within the lens barrel.