Actually, you can do whatever the hell you want to your printer. What you can’t do is make a printer that infringes on someone’s IP and sell it. You can make your own enclosure for your own printer and it can be identical to any and all IP/patents in existence and it’s totally legal.
The Sigma appears to take well to a DIY enclosure. One could cut acrylic to match the flat panel areas on the sides and front. With appropriate clips, perhaps 3d printed clips, it’s possible the sides and front could be slide-on, slide-off. The top of the printer is also mostly flat and needs only clearance for the bowden tubes and other extended items. I’d try to build a foam plug of the appropriate dome shape to see if I could vacuum form a thin acrylic dome. It would be tricky to do so, as there’s a great variation over a short distance. Even if the top cover is simple rectangular acrylic, it would complete the enclosure.
Benjamin, if you aren’t sure about getting the Sigma, you’re welcome to order one to ship to me and I’ll give it the once over and let you know how it goes. 
This guy has some reviews. http://toms3d.org/2015/11/13/feature-breakdown-the-cel-robox/ That one is for the cel robox.
Unless they’ve actually released the dual head, the cel robox isn’t a dual extruder. It’s a single extruder with 2 hot ends… 1 with a fine tip nozzle for perimeters and 1 with a larger nozzle for infill
Disregarding the material content (mostly), I’ve found I enjoy Tom’s reviews of just about everything. He provides opinions and backs them up with solid factual material.
@Justin_Nesselrotte the heated enclosure patent is about something specific, a dual axis bellows system to contain the heat to the build volume and protect motors and the extruder system from intense heat. Some Stratasys machines can get the build volume very hot, I’ve heard as much as 190°C, something you don’t want to do casually.
I know someone that says the Flashforge Creator Pro does well with getting ambient air hot enough for ABS without adding an air heater.
@Jeff_DeMaagd The patent for the enclosure is irrelevant to whether it is one or two extruders, even if those axii are independent. Also, the Stratasys uPrint line heats the build chamber to 100C by default, and that’s hot enough that I definitely wouldn’t want to deal with 190C.