or you can put electrical tape around the threads on the bottle. This helps it to actually tighten, its worked for me so far.
What if you took a small air compressor tank and used that? Given you had the space to put it, wouldnt it help to keep your engine cooler because of more coolant that can flow through?
You know those hoses that come off the water pump that go to the bottle? What if you replaced those with copper, wouldnt that help to cool the coolant even more?