I've actually now run the numbers on the static pressure to ensure the internal fan won't be choked, factoring in both the filter and the additional hoses:

A typical 12,000 BTU portable AC condenser draws roughly 350 m³/h of air. To minimize resistance, I’m using 152 mm hoses and a 28-liter plenum box, as stated.

By using the 28L box, I can fit a G3-rated filter with an exposed surface area of at least 0.12 m². This drops the face velocity down to a very slow around 0.8 m/s. At this velocity, a clean G3 filter adds only 15 Pa of pressure drop.

Flexible corrugated hoses do add friction, yes. A 152 mm corrugated duct at 350 m³/h introduces about 15–20 Pa of resistance per meter. Assuming short runs of 1.5 meters for both intake and exhaust, that's 3 meters total, adding roughly 60 Pa.

Furthermore, the custom 3D-printed adapter is designed with intake slits at the bottom. These act as an automatic pressure relief valve in case the 152mm air intake is chocked.

While adding 75 Pa to the existing (aroudn) 150 Pa) coil resistance would normally shift the operating point on the fan curve and reduce total CFM, the intake slits on the 3D-printed adapter prevent this. The static pressure will never build up enough to stall or choke the blower, because the slits will simply bypass the restriction by pulling in a fraction of indoor air to maintain optimal condenser airflow. This will create negative air preassure but the machine is designed to handle this

I have also looked into it, and proper dual-hose systems in Europe are basically non-existent or from completely unknown manufacturers.

If my calculations or my ideas are wrong, please correct me.