Wasn't overly happy with the selection of wingnut designs for an M5 nut, so I made this.

EDIT: Added a low profile version for bolts that just might not reach the full size, also added a version with softer rounded knurls.

The opening for the nut is very tight, so I heated the nut with a butane torch, and gently pushed the nut into place. In the photo, I pushed too far. The melted plastic (ABS in my case) will hold the nut in place, and there is the added benefit of the melted plastic bunching up at the base of the nut, creating a lock nut type effect as the bold has to cut through it to work. This will help hold the nut in place.

The best way I've found is to place the nut on the end of a bolt and heat that, then gently push the nut into place, till it's flush with the surface. Also, gently seating a hot nut in place, then flipping it over onto a flat, non flammable surface, and pushing the wingnut down onto the nut will probably give the best results, but I haven't tried this yet.

Initially, you will have to work a bolt through the hole until the passage is cleared, but even after, the locking effect will remain.

The knurls on the part are aggressive enough to withstand some decent torque.

M5Knob.stl has enough clearance so the bolt won't rub the sides if the nut is squared correctly. M5knoblock.stl has a tighter passage for the bolt, making it rubs the sides, increasing the "locking" effect.