Loctite PL Premium
It's not a 'quick grab' adhesive. You'll have to have a way to hold the diamond plate after you have the adhesive on it and put in position. So, if the diamond plate needs to be curved to fit etc., do that before you apply the adhesive.
Once the PL Premium has dried, you better hope you never need to remove the diamond plate!
The label says it will work on those surfaces, but I've never tried it on fiberglass and aluminum. It might be best to buy a tube and try it on some inconspicuous spot. I feel sure it will work for you.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.