I was shown a very helpful way to fit a mesh by Steve Simmonds years ago. Place a soft towel on the floor, put your grill face down on the towel (prevents scratching) place mesh on top and if possible get someone to GENTLY press in centre with their hand.

The idea is to concave the mesh a little.

Fit cable ties to hold the mesh in place, use a good number to hold the shape.

Now put on bench and in turn pull the cable ties tighter and tighter to curl the mesh into the shape of the grill, it will also reduce the outside dimensions and help avoid scratching when fitting.

Now refit whole assembly to car ensuring your happy with location and security.

Finally cut all the cable ties and remove. The mesh with spring outwards slightly and be secure in place. I have fitted a few for owners using this method.

It can also be used in reverse to secure the mesh to the grill if a grill needs removing!

BR
Colin (all credit to Steve S for this method)