What kind of humidifier do you plan on installing? There are flow through, bypass and steam and all install differently but the blue box you drew isn't a good location for any humidifier.

A flow through should be after the furnace in the supply duct but you have no plenum that I can see to install one so I'd avoid this type, you really have no room for one and the bypass or steam would be a better option.

A Bypass would mount on the return side near the top of your first purple arrow with the bypass duct running into the supply plenum, looks like enough room.

A steam humidifier could hang anyplace strong enough like the back of the air handler or even the wall studs to the left. The steam tube would then run into the plenum and looks like there is enough room.

The blue box would be fine for the UV light, (if you really wanted one) but not a good choice for a humidifier and running the moisture through the fan motor, heat exchanger and A/C coil for obvious reasons. A drain, water supply and power are required for all three types with a few exceptions.