Well think back on the original fabric.
It's made from handspun yarn, it takes seven spinners to keep up with a weaver on a loom.
Imagine doing it all by yourself at home....would you cut that fabric if you could avoid it ?
I wouldn't, and I have spun and woven my own wool.
Warp weighted looms produce a cloth with three finished edges, and one edge of fringes.
Fringes on a garment are a good thing in a wet climate, they help to drain and air away water.
So, that warp woven piece could be taken to be the 'first' principle sort of thing.
A circular cloak looks good, but the reality is that all the warmth is at the hems and not at the body.
What we do get is a long rectangle, that has it's corners neatly cut and hemmed, so that they don't drag on the ground. That looks 'circular' in illustrations, but it's really not, it's just rounded off.
I have made a cloak that had a 44' hem. I jest you not, that's what a full circular cloak ends up with as it's hemline. Drove my friend nuts because she wanted the cloak but I told her when I cut it out and sewed it for her that the hem was her job
It looked absolutely beautiful on though.
To make a warm cloak, it's worth pleating the fabric across the shoulders and letting it flow from there. Extra warmth and the cloak is much better balanced.
In practice a collar on a cloak is a very good thing; it gives a structure that sits 'on' the shoulders rather than all that weight constantly pulling it off. It also gives a better structure for attaching a hood.
I need to find the sketches I did and posted years ago showing the Birrus, and basic cloaks, for illustration. I'll see what I can find in the search.
Modern re-enactor's wool (probably the stuff that fellow doing the pilgrimage wore) isn't good barathea or loden wool. It's very good, but it's not brilliant....so usually if the cloak is to be worn in reality not just for show, they make it up with two layers. That really adds weight. I suspect that's why his cloak was so heavy.
M