A couple of people at the barn where I board had the cashel model. I never really got that close to see how it was put together, always just saw them riding with both pieces on. I have the amigo fly rider which my picky TB mare loves. The 1/4 sheet section has a cut out under the saddle. The neck section attaches to the front of the 1/4 sheet. I have never taken mine apart(always use the whole thing). I think if you took it off it you could tie it to the front d rings of a saddle as it velcros together on either side of the wither where d rings would be located.
I have both and you do not need back D rings.
The Cashel one comes with long enough strings, which I attached either round my stirrup leathers or the small leather tab where I slide the excess stirrup leather thru.
I seem to catch a number of large biting flies going underneath the Cashel one, which doesn't happen with the Amigo.
The Amigo has a seriously long taiflap, which I find rather annoying.
The cashel sheet is much stiffer and the one lady at my barn feels like it spooked her horse while she was trail riding. Sheet kind of brushed a bush or tree and moved weird on the horse and he spun and dumped her. I take it with a grain of salt though as the horse is a bit naughty and she doesn't make him behave. I live in humid Va and I don't find my horse gets overly hot with it (amigo fly rider). It sure is nice not to have a fidgety horse when riding out on trails. If you feel like you are riding in a renaissance fair, you should see us. My mare hates bugs on her head so she also wears a Cashel long nose fly mask and has a western style horse hair fly swish on the throat latch of her bridle. She is so funny when I put the swish on her for the first few yards of the ride she flips her nose up to test the swish.