How to align mesh accesories?

I am making custom mesh pieces using the Blender Roblox Rig and want to be able to align them exactly to their corresponding Rig part. However I don’t know how to make it aligned exactly to where the corresponding Roblox Rig’s part is positioned. Is there an easy way to align these without fiddling with them down the the millimeter? I seems this is somewhat related to Roblox resetting the mesh’s origin to its’ calculated bounds. Here is a picture of how I modeled it and how it attaches ingame using an Attachment positioned at that RightLowerArm’s center. My attachment (red) is shifted slightly forward compared to the default arm (blue) which I don’t want.