There's really not much to one; you could probably make one up pretty easily. If I were making one, I'd buy higher yield bar (1045 or so) such that I could go a thinner on the bar size and save some weight. But I wouldn't hold my breath on one working too well. When I bought one of our JD #9's sickle mowers several years ago there was a windrower attachment for it at the same auction, selling right after the mower. I started bidding on it because it was one of the main reasons I bought the mower (we already had 5 or 6 sickle mowers and didn't need another). But a chap next to me put his hand on mine as I was raising it to keep bidding and said, "I wouldn't do that", (or something like that). Surprisingly, it turned out he was the owner at the sale. He told me he didn't want to see a young guy waste his money. I talked to him a little about it afterwards. He said it worked ok... ish... kind of... in the perfect conditions... if you held your tongue just right... But much of the time it caused too much plugging and crop hanging-up to be worth it, and he said the same thing as ET above: a heck of a lot of weight on a 7' bar.
He said for the little amount of swathing he needed, he ended up just cutting with the mower and raking it. He said it caused a lot less frustration with only marginally more grain shatter. I can't remember what he had for a rake, but probably a simple rolabar rake. With a newer rotary rake and if you were able to rake with the dew on, I suspect you'd get even less grain shatter. It certainly wouldn't be perfect, but for any amount of swathing you expect to do with one of those attachments, I suspect you'd do just fine with a straight bar and a gentle raking.
Obviously my thoughts don't count for much as I never ended up buying it. But my opinion: If you want one for interest's sake or for giggles, you could keep looking or build one. You may even find it works ok. But if you're expecting/needing to rely on it for decent swathing, you might want to have a backup plan. Or just use a rake as gently as possible after mowing.
If I were making one, I'd build one of the centre-delivery style (second pic below). I don't know for sure, but I suspect they're less likely to plug with the centre-delivery style rather than the end/side delivery, as you're moving the crop less distance 'across the grain'. It would also probably take some tweakifying to get the angle and spacing right: You'd want enough gap between bars to have sufficient stubble poking through to positively move the grain. But not enough gap to lose material through them. And a gentle enough angle to provide a gentle turn, without the crop having to really turn sideways.
No matter how well it's designed, I wouldn't hold my breath on it working perfectly. If those attachments were truly effective ways to swath a crop, they would have been a lot more popular than they were. The video below (below pictures) is the only video I've ever seen of one. In that video, it actually seems to be working surprisingly well. But you can see a few times it comes close to plugging. If that crop were a little thicker, or less consistent, or had a little green/toughness to it, or on slightly rougher ground, you can definitely imagine it plugging.