I have just fitted mine today to a newly sprayed engine cover.
I jammed the two grilles in place using a paper wedge up against the Banks lid strut which conveniently goes side to side at the exact place.
I then squirted black silicone sealant into the 4 corners from underneath, and then went around the whole perimeter with the mastic gun, smoothing the sealant under the grill with the index finger.
I will leave it for a week before removing the paper wedges, and hope they stay defying gravity.