I don't think a working model of this would look much like a cannon. Nukes don't directly produce (much of) a shockwave; most of the shock comes from everything in the vicinity of the warhead absorbing a massive dose of prompt gamma and/or loose neutrons and suddenly deciding that all its atoms really need to be over there. So if you had a payload backed right against a nuke, even if it managed to survive the explosion, it wouldn't convert much of its power into velocity; Orion gets its power by vaporizing the outer layers of the pusher plate or a layer of reaction mass sprayed on it.

But it might be possible, nonetheless. The thing I have in mind might look something like a large chamber full of water with a nuke in the center of it, connected by some plumbing to the launch tube with the payload. Initiate the nuke, the water flashes into steam, the expanding steam drives the payload. Tricky part would be controlling the acceleration for a (relatively) smooth launch with minimal wasted energy.

(And, of course, you're left with a giant plume of radioactive steam that you still need to deal with.)