Take the flex duct loose from collar connectors, measure diameter of the collars and go buy four 90* elbows for the four collar connectors.
Attach them with 5/16" head sheet metal screws and wrap foil tape around that 2-3 times.
Get NEW flex duct of correct size.
The 4 ells are A, B, C and D .
Stretch flex duct tight between A and B and do same with another flex duct between C and D.
Roll plastic and insulation back, place 4 screws in the 90* ell, one on each side at base of the 90* turn.
Run spiral wire and insulation/plastic up over ell and hook spiral wire over the 4 screw heads, then tighten down on the wire with lip of head.
Wrap 3-4 turns of foil tape over this, pull insulation and plastic up, secure with one large or 2-3 medium zip ties and wrap 3-4 turns of foil tape.
Do same at each ell, be sure flex is tight and support every 4 to 6 feet with web starpping just barely touching flex.
This eliminates kinking, collapsing, sagging and is best for using flex.
Secure belly and skirting.
Old coveralls and tape at ankles and wrists and a old boggen and safety glasses.
Headlamps are wonderful here.
Large cardboard to lay on. Put tools and supplies in old satchel, backpack, etc and pull to you with rope.
Lay the ells and flex duct boxes on cardboard and slide it ahead of you.
Take care and best wishes,
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?