I had the same issue and asked the same question when I mounted my Frigoboat evaporator. Is you evaporator made by bonding two sheets
together, leaving channels between the sheets
for the Freon to flow? That is how the Frigoboat is made.
I asked this question of Frigoboat and the answer was yes you can drill, BUT. If you drill well away from the Freon channels then 99.9% certain there will be no problem BUT strictly speaking you aren't supposed to drill holes in the evaporator and it could possible void the warranty..... or not?