This does a great job of blocking the glare that comes in between the mirror and the tips of the visors.

With this, you almost never need to swing the visors around to follow the sun.

It does block a small amount of glass, but I think that part is legal to block - note the index marks on some windshields, and note how far the tinting goes down the glass. This matches that pretty well.

Cut a piece of card-weight material 17" x 5.5"
(42-43 cm x 13.5 - 14. cm for Euro-folks)

I used black material from a report cover, taping together 2 pieces to get the length.

Slide it up between the windshield and mirror,
till it hits the center visor clips.
Center it so each side meets and is just under the 2 sun visors.
Stick into place with tape, up against the headliner.
Maybe stick on with Velcro material later.