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Adjacent routes on metal2 + wide routes on metal1 & metal3, all connected to GND can make a good all-around-screen. But of course create massive cap losses.
I wouldn't advice it. If your differential clock has good symmetry - both regarding drive strength and fan-out - crossing signals receive opposite stray signals which well cancel out. Nearby sensible inputs could be screened individually.
You can use normal shielding and co-axial shielding depending upon the critical on those diff nets.
1. Normal shielding -- side by side
Metal2 is you conductor
Add M2 both side of the conductor with 1:2:1 spacing.
2. Co axial
Adding M3 on top sides with width to cover fully the conductor signals
Adding M2 on both sides of the conductor and also drop VIA3-2 to connect M3 and VIA1 to connect M2
Adding M1 on below sides with width to cover fully the conductor signals
Here also you can maintain the 1:2:1 spacing between the shield and conductor.