Booleans not working... again :(

This has to be basic but i just keep going in circles. I have a shape i made, padded it and got it exactly the shape i need. I had to change the angle very slightly, so I created a plane, rotated the right side piece to where i need it. Now I am trying to fill in the gap between "left vent" and "right vent", say with blendcurve or some such so that it is then one piece again. I have tried to join, union, blendcurve, convert to solids, compounds... and it still will not work.

In the end, I need the 'Vent001' to act as one solid piece so I can then apply 'Triangles', slice apart and fuse.

I had help previously with this and it worked out perfectly. but now I had to go back an change a few things and once again i am beating my head against the wall...

I appreciate anyone who's able to tell me why I suck at this, and guide me in the correct direction.

https://www.transfernow.net/dl/20250319bPBssaVB