I didn't see this question asked after my search but I have a wall in the kitchen that has nothing against it and we are wanting to add some cabinets but towards the right of this wall, about 5' from it, is a vent in the floor. I was wondering if a lower base cabinet can be sectioned/front-cap removed or if there was a special fitting (such as a 90 vent cover just for the base of cabinets) to allow ventilation from a vent in the floor? We do get vary broad climate changes so I was curious to know if I do add a vent in the bottom base would that possibly warp the cabinet even if I insulate it?
Yes, it can be done.
Relocating the vent, however, is a better general solution because:
The HVAC system can operate closer to its design. Reduced size registers and additional bends change the back-pressure and can readily throw the system out of balance.
Specialized registers and other parts are not required.
The less common installation is more prone to errors than typical industry practices.
None of which is to say that this is or is not the right solution for any particular installation, but rather than it is really only the preferred solution for a set of edge cases.
You might need to go to a specialized HVAC supplier (maybe grainger) but you need a shallow 90 degree going to rectangle kickout. Basically you want the number of square inches from the incoming vent to equal what you will fan out using the toekick of your cabinets.
Not sure you would find exactly what you are looking for at big box. The worst case scenario is that you buy a shallow 90 and bend duct to meet your needs. I would really look into HVAC suppliers though and paying $30-50 for something custom might be worth it. Think about the work and cost of relocating or doing something else. And also the fact that the vent was probably put in that location for a reason.