A mat will do absolutely nothing to contain static. Lewm you answered your own question. The electrons are coming from the air. The stylus rubbing the groove enhances the transfer of electrons. Because diamond is not conductive there is no path to ground.
I have owned two Linns in the past. All you have to do is put felt pads at the front corners where the cover contacts the base. The cover should not contact the base anywhere except at the hinges and the felt pads. (you can get them at Home Depot.) I suspect the reason why people think things sound better with the dust cover off is because the echo they get adds a tube like distortion and a false sense of space. If you use the dust cover and pay attention to image specificity you will notice that the individual instruments are better defined. Just play a good recording of a string quartet. I use ESLs and subwoofers. The system is very sensitive to just about anything. Now even if you think there is a slight degradation in sound quality, record preservation is more important. When you play a record without a dust cover and no path to ground dust is pulled into the record rapidly where it is ground into the vinyl. This is a great process for the record cleaning industry. A conductive sweep arm costs 20 bucks. If you have trouble setting it up message me. Play some new records with the dust cover on and a sweep arm. Watch what happens. No static build up, no dust, records stay perfectly clean, no build up on stylus. You might have to clean all your old records once to see the same effect on them but you will and you will never look back.
The problem comes with turntables that are not suspended and vibration of the dust cover is passed directly to the structure that has the platter and tonearm attached. Making a usable dust cover for these tables is a bit more complicated and expensive but it can be done as I talk about above. If I ever manage to get a Dohmann Helix I will have to go through that maneuver as the whole darn turntable is suspended.
Another advantage of using a dust cover all the time is limiting exposure of the tonearm to dust. Dust is the enemy of delicate bearings that are not sealed. Interestingly this is the one area where uni pivot arms have a distinct advantage not that I would ever buy another one.
I have owned two Linns in the past. All you have to do is put felt pads at the front corners where the cover contacts the base. The cover should not contact the base anywhere except at the hinges and the felt pads. (you can get them at Home Depot.) I suspect the reason why people think things sound better with the dust cover off is because the echo they get adds a tube like distortion and a false sense of space. If you use the dust cover and pay attention to image specificity you will notice that the individual instruments are better defined. Just play a good recording of a string quartet. I use ESLs and subwoofers. The system is very sensitive to just about anything. Now even if you think there is a slight degradation in sound quality, record preservation is more important. When you play a record without a dust cover and no path to ground dust is pulled into the record rapidly where it is ground into the vinyl. This is a great process for the record cleaning industry. A conductive sweep arm costs 20 bucks. If you have trouble setting it up message me. Play some new records with the dust cover on and a sweep arm. Watch what happens. No static build up, no dust, records stay perfectly clean, no build up on stylus. You might have to clean all your old records once to see the same effect on them but you will and you will never look back.
The problem comes with turntables that are not suspended and vibration of the dust cover is passed directly to the structure that has the platter and tonearm attached. Making a usable dust cover for these tables is a bit more complicated and expensive but it can be done as I talk about above. If I ever manage to get a Dohmann Helix I will have to go through that maneuver as the whole darn turntable is suspended.
Another advantage of using a dust cover all the time is limiting exposure of the tonearm to dust. Dust is the enemy of delicate bearings that are not sealed. Interestingly this is the one area where uni pivot arms have a distinct advantage not that I would ever buy another one.