Collage, sculpting, or molding: possible fillers and/or mediums?

​Hello All,

I’m in the think-tank stage of a new project. I want to create a (mono-chromatic) “painting” whose main visual interest will rely on the sculpting of a series of low relief images (think a modern version of Ghiberti’s doors). It will be executed upon a number of small hardwood panels which, when assembled together, will create one large composite image, of say 3 1/2 feet x 5 1/2 feet or so. 
My main question is what should I use for my sculpting material? In the past I have tried:

Acrylic molding pastes (on HDF) are OK, but they dry too fast and I have found them to be both too coarse and way too heavy. 
I’ve done some low relief sculpting (on HDF) using traditional gesso (pastiglia). It was OK, but was very consuming to create even a small, subtly reliefed surface. 
I have created collages using pieces of fabric, attached to either plywood or MDF using a strong solution of hide glue.

Of the three collage is very good. I like it since it’s lightweight and can be manipulated pretty quickly and easily. However, for low relief “sculpting” it’s just a little too coarse and hard to control. So now I’m searching for a (thickening/filler) collage-type substance that might be added to an adhesive/medium which I could use for some low relief sculpting an image (or series of iamges). Thoughts anyone?
I’ve wondered about creating a putty using a recipe of sawdust and hide glue but I’m unsure for a number of reasons. Traditionally framers have used a substance called “compo” that appears to be an esoteric and toxic recipe of fillers, oils and resins

I’m currently conceiving of chopping up many small pieces of linen fabric, running them through a blender, and boiling them with baking soda to create a paste-like substance has traditionally been used to make rag content paper.