@Jim_Hatch I have spacers that prop the lid open 1/8 of an inch. I don’t know if there is a direct path out that space but I suppose I could add a strip of metal to block it. However I see some of the newer units have vents cut in the front of the lid. Seems like that could be really bad.
@Vince_Lee the opening is what changes it to a Class 4. I’ve got a row of fans cut into the front of my box so there’s a possibility of reflection through the fan which does the same for me. The likelihood is extraordinarily small that we’ll get any laser reflection out of the box but because it’s not sealed the chance is not zero so Class 4 it is 
@Jim_Hatch Hmm. I guess with the original half moon hole in my lid for a handle it was already a class 4 to begin with.
@Vince_Lee I have a small half moon shaped finger cut out on the lid of mine too but there’s a lip on the lower panel that the lid closes over so the cutout isn’t exposed from the inside of the machine. If yours is open to the machine’s internals then you’re right it’s class 4. On the other hand I’m generally suspicious of any claimed “certification” by these manufacturers considering the claims for laser power and some of the wiring I’ve seen people get with their machines.
@Jim_Hatch out of the box my unit worked okay but I wouldn’t have classified it as Class 4 or Class 5… Class Action Suit more like it.
@Vince_Lee I understand people’s frustration with the K40. It’s not a consumer commodity product. At best it’s a tinkerer’s box. Not unlike my first PC - a Commodore 64 with a cassette drive.
For a properly supported, professionally built laser cutter there are many options out there - ULS, Epilog, Trotec, etc. They’re not $400 though. Or $4,000 either. $10K is about the going entry price for getting into laser cutting and engraving. (An entry-level Epilog not much different in capabilities than the K40 is $8,000.)
Anyone who expects the quality of an $8,000 machine in a $400 one is kidding themselves.
The K40 does what it claims. Not as easily or reliably as an Epilog but the claims are generally true (even the 40W claim is defensible). Not class action material. For that you just need to look to Theranos or Takata.
Are you guys on this post using a duct fan along with these other air devices or are you trying to avoid the fan?
Here is a post on my setup for reference.
@donkjr I have both. I have the front muffin fans to add air to the box. Otherwise there isn’t enough incoming air to keep up with the exhaust - just a small hole in the bottom and what can be pulled through the gaps in the lid & seams.