The first step would be to Drill a long enough hole in the bolt. To be honest it was done by my nephew as he has more of the power tools and he had the bolt and the nut... thanks a lot for him here. The reason I mentioned different size of drill bits you should have is because the bolt if you have a good one is strong and drilling a 8mm hole in one go could take a lot longer then drilling a 4mm hole then a 6mm and then a 8mm. But maybe he did not have the best drill, for sure next time I stop by I'll bring him some good 8mm drills so we can make more if these. 

The next step is to cut the head of the bolt off so it'll be only the threads to twist it into the nut. When that is done, try to get rid of the Sharp Edges to make it more friendly to geocachers. The best if it does not look to professional, as construction workers would cut and just leave it as it it.

Now we need to prepare the nut, add a magnet to it, the best would be a 15mm diameter one, which I did not have on hand, but already ordered so I should have it in hands for the next one. You need to find a glue that holds them together, I used what I had, and still testing if it survives the weather, rain, cold and heat. I might need to find a two component glue that could be the best solution.

The logroll I used in this one is a long piece of paper with a introduction header is really small letters, glued to a small toothpick and rolled up, then I used a small piece of a straw to hold it. As you can see on the first image I also used some thread to tie them together, but that is only to get the paper used to how it should stand, and before the container gets out for geocachers to find I will remove it.

If you need any more help or hint on making your own, just let me know.