My RSSI pin on the RX is not soldered to the flight controller, so I don’t have RSSI at all. But I’ll measure the distance after applying your advice. Thank you for them!
A plastic enclosure generally just there for mechanical protection.
It depends if the electronics need to be shielded or not. If you open up a DJI drone, you’ll find all sorts of metal covers. Metal covers are more expensive and heavier than no cover or a simple plastic enclosure.
This is one area where intuition doesn’t work. Carbon is generally not conductive when combined with other elements such as in most plastics. Carbon fiber is conductive. I think this fact surprises a lot of people when they first learn it. Carbon fiber doesn’t conduct as well metals one would normally use but it conducts enough that you need to take precautions when using it.
Linear antennas receive and transmit best from the sides of the antenna. The transmitting and receiving antenna should be parallel with each other. You want to arrange the two antenna on the 'copter in such a way that at least one of the antenna will be close to being parallel with the transmitter antenna no matter which way the 'copter is oriented. The 90 degree rule makes sure that at least one antenna will be within 45 degrees of parallel with the transmitter antenna.
Don’t forget Ardupilot has logs, you can check the logs for the RSSI value.
If you have a Frsky receiver with telemetry you won’t need a connection and will be able to view RSSI on your transmitter. If you don’t it is worth connecting the RSSI pin on the RX to the flight controller.
I think @ddegn has given you a pretty good answer.
When you look at antennas like Crossfire they recommend a vertical placement of both the RX and TX antennas. It’s not the most convenient but if you want maximum performance that’s how you’ll get it. If you’re going long range then I’m assuming you’re not doing a lot of freestyle, so looking at that kind of mounting may be worth it. But again, tough on a small build. With both antennas horizontal you will always run the risk of the carbon fibre frame getting between the TX and RX antennas and blocking the signal. If you do install the antennas horizontally, then I would use the 90 degree idea. As was explained, by having them at 90 at least one of them will hopefully be in a good position.