I have not built one, but I own one. There are several models on the market. My plan is build a prebedder that will work with my set-up. I have a buckeye tractor co combo layer. Buckeye builds their unit so that it is almost infinitely adjustable. The bedder section is easy, drip tape shank is super simple, the tape roll must have brake to prevent overrun of spool, the layer part is just rollers ( mine uses PVC sch 40 pipe with wooden caps), the stretching wheels are spring loaded with a tensioner and then the cover discs are very easy.
The other units are similar they just have different parts. If you know exactly how you want your bed you can weld it up and it will preform better in my opinion. Buckeye has two good resources on their website.
My thought process in going with the buckeye was adjustments for different beds. The mechanical advantage of moving soil instead of plowing it in the press pan. It makes bigger beds with less HP. The toolbar system they use is simple and versatile. Their knowledge is second to none on working with your soil instead of against it.
I make 32" wide beds 6" high with a Mahindra 5010. I have assembled it three times adjusting it like I want. Rainflo is a straight forward sled type that requires 10 HP per inch of height. Reddick is very similar. Kennco is a very nice design also, but requires some HP.
I would first look at the two documents from buckeye, they explain the art of doing it. The whole design is very very simple. I would have built mine if I had ever seen one up close.