The core mixture I found to come out reasonably well for core making was based on several attempts with different materials and wetness. I settled on ten parts of sand to one part of Masa Harina. The massa is a finely ground corn flour that my wife had in the cupboard. The sand is similar to Petrobond according to the people I bought it from. I made a rather dry paste of it by adding a tiny bit of water, and mixed it all together well. Earlier I tried adding molasses, without the massa, but it was a real mess. I probably put in too much.
I made a core box out of a piece of 3/4 inch copper water pipe that I split lengthwise since I wanted the core to be a 3/4 diameter by about four inches long. I put wooden plugs in the ends and slipped pipe couplings over the two halves to hold it all together. I then "cooked" the thing with a propane torch, not getting it hot enough to burn. It smelled pretty good while cooking.
The core that came out was fairly durable but would crumble if handled roughly. I have not tried making a pour using these cores yet, but I believe this core process will work well.