Not tonight I can't. I can have a go tomorrow.
Okay, "channel" might be the wrong word.
Take a piece of aluminium strip, long enough to reach the post position and still give you about five inches outside the F-hole.
Fold each of the two long sides inward so that each side has a channel big enough for your twine to run in it, but keeping the twine from escaping sideways.
Run the twine up both sides of the aluminium so there's a loop at one end.
Put the sound-post in the loop.
Hold the twine so that it tightens on the post. Now you've got a T-shaped assembly, held together because you're keeping the twine tight.
Manoeuvre the post in through the F-hole and into the correct place. Pull or push as appropriate.
When it's in the right place, let go one end of the twine. Now the post is in place and not attached to the aluminium.
Pull the twine and the aluminium out, and store them somewhere for next time.
If I knew where mine was I'd take a pic but it's somewhere in my personal chaos and it might take a week to find it.