The office admin harped on me until I made her a whiteboard holder. Plus it made good practice.
I experimented a little with this one and sprayed the cardboard with acrylic sealer first. That helped a bit to achieve the smooth surface in the middle. Cardboard is still not a great form material, but this was a little better because I used some that was not corrugated.
I also tried another incense holder using ABS plastic as the form. That produced a good surface and didn't stick to the concrete at all, but the design is still not up to snuff.