I have been trying for a long time to post some pictures of a doll bed I made for my granddaughter and have finally given up. The last straw was when the Quiltingboard closed my connection. At least that is what my computer said.
Like you, I was having trouble finding something that would be stiff enough yet not too brittle. I went with making bolster type pillows. They are basically long tubes that I stuffed with batting scraps. I sewed them seam side down to a piece of fabric that became the bottom of the bed. I sewed a grosgrain ribbon to each end of the bolster then used the ribbons to tie the corners together. Now I had four thick sturdy sides and a piece of fabric holding them all together as a base. I took a piece of plastic cakeboard (Amazon), cut it to size (a tiny bit bigger than the base of the bed so it tucked under the bolsters) covered it with fabric and tucked it into place. Covered that with a "Mattress" that was a sack stuffed with more batting scraps and tacked together through all the layers in about five places. Doll goes on that, little quilt on top. Big pillow with a ruffle all around. Hope you can picture this.