Have a trouble to cut a 5" inch of hole in the trunk. I don't have space to make hole through the side of trunk. the only space is let between joists and, from where its hard to cut a hole of 5" 
Thanks!
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1You mean without moving things it’s not reachable? Sometimes we have to make an access point cut from the inside and then seal up the access point.– Ed BealCommented May 4, 2021 at 20:43
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@EdBeal has the right idea. I would use a good mastic to seal the access pint and not tape.– DavidCommented May 4, 2021 at 21:20
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@Ed Beal, How can I make a hole in trunk which is not directly accessible. There is nothing I can move. The hole where i wan to make is almost 3 feet away from soffit's outside edge.– PowerTechCommented May 4, 2021 at 21:37
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You better call a pro then and they will show you how to make an access point when there is limited space. The idea of an access point is cutting a hole from a different location. If you can’t access a way to do it from the inside the trunk it might require pulling a section down putting the branch up then reassembling the trunk line.– Ed BealCommented May 4, 2021 at 22:25
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Can you show picture of place you want the hole. Maybe if the good people here, see the access problem they might have an idea.– crip659Commented May 5, 2021 at 0:08
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1 Answer
Some options that come to mind are using a low profile drill to start a hole and then awkwardly cutting it with tin snips or a nibbler if you have enough space. With a little more space you may even be able to fit a hole saw with a low profile drill. Some "Adjustable" hole saws have shorter length than common arbor and cups.
If you really can't fit anything in there, from your picture it looks like you have access to the opposite side, so you can get a 5" holesaw and a 18-24" hole saw extension bit and drill through both sides of the duct. Once you're done, seal the hole you don't want with a metal plate, self tapping screws and that grey duct basher goo. That gets you a hole and just leaves you with needing to screw your flange on, which could still be a huge pain, but again a low profile drill should do the job with only 5-6" clearance. If you don't have 5-6" clearance, you don't have room for your duct anyway, so I assume you have at least that to work with. There may be some adhesive that is considered acceptable, but I've never seen a professional HVAC guy forego screws.
Hex head duct screws are particularly easy to work with with a magnetic bit, but an option for really hard cases is to drill a pilot hole and then put the screw in by hand with a stubby screwdriver.
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I think your first paragraph is the right answer. If a 90 degree drill adapter is used that has the added benefit of reaching further into the joist cavity. Looking at the first picture with the tape measure it appears the center of the hole is 30 inches from the soffit. Even a larger person should be able to get their arms into the joist cavity far enough to reach the center of the hole with a drill using a 90 degree adapter with a low profile hole saw. Drilling up through the bottom is also a good option. Commented May 5, 2021 at 3:13
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I would prefer to go with 2nd option. First cut a bigger hole from opposite/bottom side to make the required hole to the top from inside. install a collar and duct to the top. then patch and seal the bottom hole with metal sheet. only problem is how to seal/tape the collar from the top. Commented May 5, 2021 at 3:51
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@VickyP actually now that you make me think of it, a 5" hole would probably allow you to screw in and mastic the flange from the inside with aircraft bits and then all you'd have to do on the top is reach over and get a compression clamp and a wrap of tape on. You'd need a significantly larger hole on the bottom side though unless you fold the flange to get it in and then somehow unbend it ship in a bottle style.– K HCommented May 5, 2021 at 4:14
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@K H. i didn't understand your last statement "unless you fold the flange to get it in and then somehow unbend it ship in a bottle style" ? Commented May 5, 2021 at 16:18
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@vickyP If you're attaching the flange from the outside, you only need a 5" hole to make a 5" hole with a holesaw. If you're attaching it from the inside, the bottom hole has to be large enough to fit the wide part of the flange that overlaps the edges of the upper hole. It's sheet metal so an option to reduce total hole size is to bend it, put it in and unbend it.– K HCommented May 5, 2021 at 22:55