All cheapo PSUs and DC-DC converters are switchmode devices and wil generate a lot of noise. The problem signal is the switching frequency, which will be in the 10s or 100s of kHz, so propagation will be mostly conducted rather than radiated. This means your metal shield will not help, but a well-placed capacitor might. In any case, the Pi is pretty resilient and I've never had any problems.
You have a made a good choice I think - I always use at least 3A for a Pi 3 with add-ons, and an adjustable output allows you to compensate for inevitable volt drops.
Avoid Du Pont connectors; they are meant for experimenting, not building products. They should take 3A but I have found them to be very unreliable. The slightest vibration and the signal can drop out, which will cause crashes if you're using it for power. If you must use them, then multiple-way connectors are much better than single ones. The best way to power the Pi is the Mini USB port, because that's where the fuse is.
Submarine communication systems engineer and amateur robot enthusiast.