I would glue it on using a suitable adhesive. The relatively thin diamond plate should dissipate the heat well enough and I doubt the adhesive will cause any interference. It would be different if you were installing a wooden skin which would work like insulation to retain heat.
Stainless steel, as I'm assuming your diamond plate is, has a very negligible R-value... it actually transfers heat very well. Therefore, I'm confident that if you attach it tightly, avoiding any airspace, you'll be fine.
My thinking first use 80 grit adding a tooth to the aluminum plate then applied contact cement with plenty of pressure. The adhesive layer would be only a few mils thick to the kezzers wavy metal in thickness for plenty of heat transfer. Even removing the paint on the keezer to bare metal would increase heat transfer. What your doing would put SHMBO over the top at my house as I already have engine parts and prizes laying around. Blend in with the house is my theme.
My keezer is skinned with copper sheet and trimmed in mesquite. I just snugged the copper up against the sides of the freezer and tried to avoid gaps/airspace as much as possible. It dissipates the heat very well.