Long time follower ... first time poster.
There is nothing like a Blackhawk shoulder mount, and nothing works as well for BID.
After trying to build my own out of gunstocks or brackets, I finally gave up. Gunstocks do not have enough "pitch" to enable you to see through the viewfinder. Most brackets and stocks are simply not adjustable enough to enable your eye to comfortably use the viewfinder.
I use my Blackhawk for more than just birds. It's been on several African safaris. On safari game drives there simply is not adequate room for tripods, or even monopods in some cases. I've never been on a game drive where I couldn't use my Blackhawk. The Blackhawk is simply outstanding for capturing live action like a lion or cheetah chase.
Yes, the Blackhawk is expensive, but you get what you pay for. The material is extremely light weight, but incredibly durable (some like Kevlar). The frame is very rigid and will support any major telephoto and camera. I use mine for my 5D III and 500 f4 lens, often with 1.4X extender. It should easily support a 600mm prime and 1DX if your arms are strong enough to handhold such a rig.
I bought the trigger shutter release. It is very unique piece of cable that hooks a trigger button in the handle to the remote shutter on the camera. While not quite as sensitive as the shutter button on the camera, I found it very possible to "half press" the Blackhawk trigger to engage camera AI Servo without firing the shutter.
Unfortunately, my Blackhawk cable was damaged in transit on one African trip. But what I found is that you can use the Blackhawk more on the center of your chest (rather than pure shoulder mount) to get an even better alignment for you eye in the camera eyepiece. When held this way, you still get a stable two hand platform, but you can use your right finger on the camera shutter.
While there may be other harness arrangements possible. Nothing beats the speed and flexibility of a Blackhawk for live action, especially BID.
If you are worried about it looking like a "gun ", the stock quickly pulls out, and when dissembled, it is very compact ... and no longer looks like any type of weapon.
While I found the response time to bit slow, the owner was very supportive in sending me a rubber cushion that was missing from my purchase.