Welcome to the forum, Collier.
There are lots of choices for the line to use on the suspension. Whatever you choose, it needs to be small enough to prussik onto the buckles nicely, non-stretch, and strong. I'm personally partial to 5/32" Spyderline
Concerning the end channel, it should be no problem just running your cording through that channel. If you use the 5/32" diameter line linked above, I would use a 'double strand' of it run through the channel. That's easy to do, since a prussik coming off the buckles will leave you with 2 free ends. Just make them long enough to go all the way through the channel, and then to tie back onto the standing end.
For the knot, a properly tied bowline knot
(using both strands) works well.