I've seen more smoke generators that convert existing smokers to cold smokers than I have a dedicated cold smoker. There are some dandy plans for building a permanent cold smoker in your backyard, one friend built a plywood one that breaks down and stores flat. I've seen some people use a large moving box, they are $3 or $4 at Home depot.
My ideal cold smoker is my drum smoker. It's 40" tall, slightly taller than a 55 gallon model, has excellent draw and draft, and a lot of volume... which is critical when using a pellet generator. It has a hanging rack for sausage, bacon, ham etc., and a regular cooking grate. With the hanging option I've never come close to filling it to capacity. I can use a pellet or sawdust generator.
When I use a pellet generator, I can even block the lid open in order to maintain a near perfect flow of smoke. Cold smoking is all about quality and delivery of smoke.
Second in line is my Mini WSM. It can hold a slab of bacon or a shoulder ham, but it excels for smoked cheese and butter when combined with a sawdust smoke generator. Hands down the best combination I've used in over 40 years of experimenting. And it's so portable I can easily follow the shade, or hide from the wind for the 4 or 5 hours of smoking time I use for cheese. The photo below is 4# of cheese, but it will hold 6#.
I agree with SmokinEdge, the A-Maze-N smokers are great for cold smoking. I get about a 5-7 degree increase from ambient temps. I prefer the the tube smokers. I use them both in the Yoder and my Fridge smoker.