Where on the gateway box is the hole to plug the IR exten...

[Penultimate]

Where on the gateway box is the hole to plug the IR extender in?

[wtwasp]

Cutting a hole in the BOTTOM of anything for heat dissipation is like cutting a hole in the TOP of something to drain fluid accumulation. 
Heat RISES. Cold DROPS. If you want to properly ventilate a space containing a heat-generating device, cut at the top. 

[jimjaarsma]

Partially correct. While heat does rise the best way to ventilate a space is by creating air flow. A hole in the bottom will allow air to enter from the bottom and then be drawn out through the top. Check out the ventilation on roofs which is designed to facilitate this. It's called convection.

[mdk]

@Penultimate -- Where on the gateway box is the hole to plug the IR extender in?

As far as I can tell, "nowhere" on my Arris Gateway.  The Arris Portal box has a well-labelled "IR Blaster" socket.

The Arris Portal MP2150 is the new generation of Portal available with your Shaw Gateway HDPVR unit.

This version is the same as the MP2000 unit,

however, it supports an IR Tethering and has a touchscreen interface on the front of the unit.

Yes, I think that it's a bad idea to tightly enclose the Gateway to prevent air circulation.