The Captain kit (a.k.a. the $15.00 bits box) has several little bits and bobs in it that make decent enough Rosarius stand-ins. That box also has a torso front that has rope draped across the chest and a little spot to glue said Rosarius bit to so that it looks like it's being worn around the neck.
Here's an example on my Deathwatch Captain I made a few years back:
Here's the torso front without paint:
And here's an example of what said torso looks like with the bit attached to the right side of the torso:
Not a great picture but hopefully you can see it ok. If you're planning on making your own plastic Chaplain then I would consider picking up a Captain box for the bits. You'll have to find a Crozius bit but beyond that you'll have a lot of cool bits for days, including a Combimelta and combiplasma.
Hopefully this helps. Take it easy for now.
-Red__Thirst-