AlDrag:

 

I just found out my return plenum is installed like this! Oversized, so they just forced it on with tape lol. I posted it on "My Efficient Electric Home" Facebook page, and everyone is having a laugh...standards must be higher in Aussie or I just got bad luck haha. They're even laughing about the wire mounting.

 

I always make an internally insulated return plenum at least 400mm long. This allows the air to equalise over the coil.
I do a special detail for these Mitsubishi ducted units as they are awkward to connect a plenum to as it has a turn in on the back, not a raw edge spigot. I get a 30mm spigot made to attach to the unit first then have the return plenum to fix onto that. Either a mez flange or slip over connection as our return plenums have either 25 or 50mm internal insulation so there is a step in the sheetmetal. Lots of duct sealant. Any bare metal is externally insulated with ductwrap as it can condense water here and drip, particularly in a hot humid Auckland summer! 

I’m sorry you’re in this situation. My company mainly does commercial work as it’s a fight to the bottom in residential in both price and quality. The crap installation we see is sometimes unbelievable then you hear what they paid and they saved about a thousand dollars.