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My current range vent does a wonky turn around and prevents the cabinet door from closing fully. I'm trying to figure out an alternate way to make the connection, but don't really have any good ideas.
My current thought is to hack up a 3 1/4x10x4 End Boot and connect to a 90 elbow straight out. Does anyone have a better suggestion?
Get custom.
Despite the tubular similarity, ducting isn't rocket science. So grab some straight pipe and a tin snips, put on some good gloves and start cutting. You could even make some template tubing out of cereal boxes to work out shapes.
Use sheet metal screws or pop rivets to connect overlapping tabs. As long as you roughly maintain cross-sectional area at all points it'll be fine.
Once you're done. Seal it up with silicone caulk and/or foil tape.
I ended up using a 90 elbow and an end boot to fix it. It took about 2 hours of trimming and positioning to get the pieces to fit together nicely. A super important consideration from using this type of boot, due to the low clearance on the right side, I had to spend some time to make sure the vent flap from the fan was able to fully open without being blocked. I used short screws to keep the boot at the correct height and prevent it from sliding down and blocking the flap.
The end boot and 90 is a good idea. Might need a straight piece first to raise it to the correct level.
You could replace the range hood with a rear venting one, so you'll need no duct at all and you'll get the whole cabinet back.
You could also replace the boot with a shorter transition boot like the one pictured here. 
This one goes from a hood outlet to 7" round (but can also be found in other sizes). That would leave you pointing straight upward and save you a turn rather than having to 90 directly off that HVAC end boot that you linked which looks more like what you'd find on a floor or ceiling register.
I find these parts are usually sold in the kitchen section of the big box depot rather than with the rest of the HVAC ducting.
Update:
I see in the attached photo that there is already a transition to vertical round duct. In that case, I'd just try to shorten that crazy straw as much as possible with snips and foil tape.
You can cut back the stub sticking out of the wall to give yourself additional room. Just leave enough stub to get tape and a band clamp around.
