No expert knowledge here, just a lucky find. Some time ago I stumbled upon a shipment of VERY thin poly fleece blankets in a discount store, they're only about 1/8 of an inch thick so I got me a pile, cost about a dollar apiece. I've cut and used those for batting for table cloths, your quilting ( mine is lousy, just learning) shows beautifully but the runner is still thin. When I run out I will get more extremely thin fleece for that purpose, the store may not have more of those precut blankets. Maybe this would work for you? If not, you can always use it , I made a flannel dressing gown for myself, used one blanket for lining. So warm and soft, it's getting cold here!

Same here.

I have used pellon fleece in my table runners and they don't make it thick but gives just nice body for quilting. If that is too much I would go with flannel.

I've used flannel on the back,and no backing,and quilt as usual. I have also used 2 layers of flannel, one for the batting and one for the backing, just to give the quilting more definition.

I agree with everyone on the type of "batting". Also, another thing I do alot with table toppers and runners is to press seams open. Helps the item to lay smoother.

Just be sure to shrink your muslin first. Otherwise with will give it a crinkled finish. Unless that is what you want. I use flannel for all my table runners and it is a little like putting batting as far as thickness. But it does work really well and lays real flat.

I have used just a piece of flannel between your layers and it lays very nicely

If I don't want to add bulk, I usually use a layer of flannel in between. Well washed and dried to preshrink it of course.
But, I'm almost converted to another product.
Micro fleece.
This stuff is almost indestructable, super soft, and not bulky. I'm not sure if it's the same stuff as the minky that people talk about.
It is stretchy though. You have to baste it down well before adding the top layer.
I just finished a bargello using this for batting and it turned out fantastic.
And - best of all, it's cheap in comparison to other options. I just buy a blanket at WM and use that.

I would use Thermore (made by Hobbs) 100% polyester, especially made for quilted clothing, wall hangings, miniature quilts and other small projects like mug mats, table runners, etc etc. It is wonderful!!!! Gives just the right amount of loft for a smoother and thinner look.

There are no rules.

You might like to make a mini sample like the top you have now, and use this sample to try several ways - with the dark backing only; with a layer of muslin; with a layer of flannel; - until you find what you like.