A few people have mentioned size.
The prototype ergonomic strap is adjustable between about 39" and 50" or so.
Of course, your actual strap, should you choose to order one, will be made to your exact measurements. But, because everyone's a different size and shape, the proto strap won't be exactly right.
However, it will give you a very good idea of whether the ergonomic strap is the thing for you.
Karl