Can you safely use a cast-iron grill pan as a heat diffuser?

We recently got an earthenware tagine. However, being earthenware, it can't be directly exposed to the stove flames. The manual calls for a heat diffuser, but those are apparently impossible to find (there's a store 45 minutes away in another city that might have some).

I know you can use a cast-iron skillet as a heat diffuser in a pinch, but we don't have one large enough. What we do have, however, is a large cast-iron grill pan. I initially thought that the ridges inside it would be a problem, causing uneven heating, and thus dismissed that idea -- but after some searching it looks like the overwhelming majority of heat diffusers actually have ridges, domes, holes, or other surface features that would produce uneven heating anyways!

So...can I safely use a cast-iron grill pan as a heat diffuser? If not, why not? In either case, what's the purpose of the various surface features on heat diffusers that prevent them from uniformly contacting the cookware on top of them?