I use a Nikon 300mm f4 lens. It's a good lens but it's also rather heavy.
What is the safest and most convenient way to switch teleprimes (or heavy lens in general) out in the field? I do have a Lens Flipper but I doubt that would work well with a heavy lens. A cotton carrier is an idea but I don't know if it will work well when I sit down.
I don't have an extra body so carrying a lens on another body isn't an option.
From the sound of it, you are carrying only a body with a lens attached and the 300mm. The camera is presumably slung over your shoulder, around your neck, or in-hand, and so you need a way to also manage the 300mm but you don't want to pull out the big camera bag, correct?
The Cotton Carrier system may be a good idea. You would need to carry/wear the vest or belt, obviously, and as you said it would likely get in the way when sitting. I don't know if it would be a concern, but it would also not offer any protection from your surroundings.
I would look at a small bag such as the Nikon CL-M2 that comes with the 300, or the Think Tank Photo Lens Changer 75 as a way to carry this over my shoulder (making it easy to manage when sitting and offering some protection). In fact, that's exactly what I did when carrying a 70-200 f2.8 around, which is about the same size as the 300 f4.
I moved away from that approach, though, and now use a Think Tank Photo Skin Body Bag for "light" travel because it's a little bigger and can hold a lens and flash, and has a few pockets for an extra memory card and battery.
Also, while the 300 f4 may be the biggest and heaviest lens you have, I would encourage you to just use it more to get comfortable with the weight. It's not that heavy and carried comfortably and as part of a larger kit I expect you would quickly get used to it.
I'd suggest something like the LowePro S&F Lens Exchange case. There are similar ones. They hold a long lens, and when opened produce a pouch you can place the other lens in, sort of like a bellows. You can attach the lens and detach the lens while it sits in the bag (shorter ones not so much, but they are handy for just placing on. Look at the videos to see how it works.
I tend to carry the heavy lens on the camera (a sigma 120-400, so quite heavy). I use a modified QR plate on my camera rucksack belt to take much of the weight, and have a slightly springy strap.
My wide zoom can go in a lens pouch on the shoulder strap (the bag is a lowepro). I tend to want the long lens ready (for wildlife) and be able to stop when I want the wide lens (landscape/macro), so this works for me.
I keep heavy lenses on my Black Rapid and keep smallers lenses in my bag. I can quickly switch. I toss an extra back lens cap and body cap in my bag just in case. I think the bag I use the most is a LowePro Flipside 200.
