I can recommend heavy duty velcro strip. It is self adhesive and is very good at absorbing vibration. I use the 2 inch wide version and cover the whole of the flat area from directly below the light lens.
Trim the tape to the shape of the mudguard, offer up the plate and trace around to match and apply the opposing velcro to the plate back. Cross lay the 2 sections, ie top to bottom on one and left to right on the other. It grips very strongly and it stop you pulling the velcro off the mudguard or plate.
A 2" x 4" patch is rated as being able to hold 15 lbs. If you do crash, it will detatch on impact rather than breaking the plate and you can just press it back on. Any dirt build up can be brushed off with a stiff brush to restore the grip strength.