Over on FB, a guy is rebuilding his engine from a nut that dropped into his intake valve. Where did it come from?

Conclusion. Buy a ZX14r new. Check it over the first day. If someone else works on it, check to see if the wellnut is in place.
An owner who bought a 2012 new, let it sit for 13 years (injured?). Last month his Bike was leaking fluid from everywhere. He found out the Master Clutch Reservoir did not have the rubber diaphragm inside.

More discussion here....

https://www.facebook.com/groups/186546582100481/permalink/2307715979983520/

How about an all Nylon insert? Who makes one? 3D Printer?

YEP, if you really know you can't wrench, you better make sure whoever does is knowledgeable and definitely going to do a thorough and professional job.

As soon as I started reading your post, I knew which well nut was going to be the culprit. There were so many important improvements to the Gen2, I'm amazed they didn't address this. All it would take is a non-well nut fastener with a bolthole. Put a rubber cushion under the bolt head and it would do pretty much the same thing. Suzuki does this.

In my experience, it would be very difficult to push that well nut through the hole so it drops into the air box. It's also safe to run with no bolt in the well nut, other than the fact that leaves a little hole where non-filtered air can enter the air box. There's never enough vacuum in the airbox to suck the well nut through even without a bolt securing it. The air box has pressure if anything, not vacuum. The danger comes from over-tightening this bolt. The well nut contracts and splits against the edge of the hole it sits in. The interior portion with the nut in it could fall down an intake or it could bounce to the back of the air box in the water drain. If it goes down an intake, you can't start the engine. IMO, anyone would know if the interior portion of the well nut fell off because it would only fall off after removing the bolt that had been overtightened. That was my experience, the interior part of the well nut held by a small chunk to the outer portion. After the rubber had split, the nut turned to the side so there was no chance of grabbing it with a bolt and pulling it out. Fortunately, the connecting chunk was strong enough, I pulled it out along with the outside portion. From that day on, no tightening that fastener, I just barely snugged it up. The bolt fell out a couple times but that's worth not having to deal with the problem caused by over-tightening. That's one fastener to check often on the bike. The rivnut idea is good. I don't know how Kawasaki didn't just cast a bolthole bung in the air box and put a grommet in the fuel tank bolthole.

When the first Well Nut is lost no one is going to use a Kawa $6 replacement. They are using a cheap 6M replacement. Maybe with a thin flat shoulder. 10 for $7 from AZN.

Use E6000, or Amazing Goop Glue... same shit. Just a little dab will do ya.

a blind rivnut is the right solution. M8 size. M10 seals the airbox in conjunction with JB tankWeld, but you probably dont need it air-tight. The OEM wellnuts are good quality. Thick and sturdy. Maybe $10 to get a pair. I'd go OEM or rivnut

Ya, I've only used OEM well nuts other than those in the windscreen of my Gen1 which were sourced from Pro-Bolts. The Pro-Bolts well nuts were good, all others were OEM and on the bike when purchased new. All lasted perfectly fine other than the air box well nut at the corner of the fuel tank cover. I replaced that with an OEM well nut.

On my brand new 14, I'm finding the well nut fasteners are barely tight at all and none seemed to have loosened. I think over time, they need a bit more tightening. The rubber contracts and provides less pressure against the head of the bolt. One thing though, if you tighten the well nut fasteners TIGHT, that well nut gets severely squashed and the fastener now needs to be tightened to stay in. Do that with the air box well nut in question and it will split. Well nuts are a great innovation but there seems to be a bit of an art to tightening them. Rule #1 is not too tight and the other rule #1 is NEVER use Locktite on them. ....so your kind of at the mercy of the spring left in the well nut which is where the art comes in. The fasteners that needed frequent checks on my Gen1 were the front fender bolts and the air box bolt. The fasteners under the seat that held the tail pods on were not well nut fasteners but they seemed to eventually brake the tabs, not from tightening but from vibration, I guess. I never figured that out....get new tail pods in advance, I guess, the tabs are going to break and they're impossible to mend....plastic welding kit maybe?