ductshvac
For sheet metal supply/return plenum boxes, I noticed the builder didn't make holes in the top for any of the ductwork. I figure it's because it wouldn't make sense considering every duct works its way back down from that point. Question I have is for cases where I just have run out of room around the plenum box to make new duct, is it ok to make a new take off on the top of the box? Is there any gotchas with doing that? I figure not but just checking.
You need to know how to calculate the required CFM - cubic feet per minute - that your ductwork will allow. On a room per room basis you need to determine how much air flow is needed, how large the vent needs to be, and thus how large of a duct you require. Then you need to determine how large of a trunk you require and at what points you need to reduce it after branching off legs from the trunk.
Finally you need to also know where to position the return air and how large it needs to be, in order to support the output from your HVAC.
I don't know how to calculate those things - I just know that's what you need to know.
If you can't get to both sides, you could drill a 1/2" hole (which may be tricky) and then use a knock out plug that is designed to snap in from one side.
You could then use something like the strain relief described in gregmac's answer to mount the thermo couple on the plug and then snap it in and seal the edge with foil tape.
Best Answer
The reason there are no supply ducts running from the top of the supply air plenum is that the air from the fan in the air handler hits the top of the plenum forcing air equally to the supply runs coming off of the sides of the plenum. If you were to have a supply run coming off of the top of the plenum it would get more air because of it having the path of less resistance. Return air plenums do not matter where the duct openings are as much.