If you don't hear good advice from someone who's actually done this, I suggest:

1) Contact the factory for advice.

2) I suggest using a side cutting bit on a router.  A dremel based tool could be used through the thin aluminum but this might be slow with the bit quality dubious.   I would go to the trouble to make an outside pattern out of 1/8" plywood, but this is me.  Locate and fix the pattern exactly on both sides and cut through the skin.  Put tape all over so you don't scratch the painted aluminum with your cutting tool.

TM bonds wood pieces between the inside and outside skins.  Look at how the vent cut-out is prepared.

I noticed that new TMs [at least the 27s] have the fan now toward the door.  I'm curious as to why [other than being more central in the unit].