The Yakima Shear Block # Y8810006 you mentioned are made out of aluminum, and the Shear Block Bases # Y8810007 are made out of a tough Zytel nylon.
You may be able to modify these to secure your Yakima round bars to the side of your trailer, but these towers were designed to attach to rain gutters like what you would find on some Jeeps. You may be able to modify these components to allow them attach to a flat surface but I do not have any information on what would need to be done.
To... view full answer...
The shipping weight on the Draw-Tite Max-Frame Trailer Hitch Receiver # 76209 is 37 lbs. If you are planning on towing or carrying a bike rack, I would be happy to recommend a bike rack, trailer wiring, ball mount, and tow ball if you can let me know how you intend to use your hitch.
Included is an installation video for this hitch on a 2019 Subaru Crosstrek. view full answer...
The lock and keys for the Inno Ridge Rooftop Cargo Box # INBRA220R-BK are made of metal; zinc plated steel. Given enough exposure to the elements they could corrode over time but the locking mechanism is decently covered to help prevent that. view full answer...