I took a look at the specs, and man, they really made that thing TIGHT! So your co2 tank is on the back of the fridge and the line feeds through to the coupler. And you have a larger than necessary hole to fill? Is it the factory condition?
Either way, I think you should fill the hole with some insulation -- anything would work, batt insulation, putty, spray foam, chewing gum...whatever! Then you could tape over it if you wanted, but the main thing is just to keep the nice cool air inside from leaking to the outside. It's more of an efficiency issue than one which will affect your keg temp or beer pour. Big picture I wouldn't worry too much about it...
Congratulations on your purchase! Before you know it, you'll want to start home brewing if you haven't already. Then you'll need a double tap head. Then...it goes on and on : )