Nuts and bolts are simple items to mould and cast.
Typically with a bolt you want the length of the master bolt to be longer than the finished cast part. Reason is, the bolt will need to stand strait up to mould it. To get the best seat you want to be able to attach that bolt into the base on the mould itself. Just as it seems, as if the bolt is being bolted to the ground but just a few threads deep. Clay the base around the bolt threads to seal it to the base, done. For one bolt a simple cup mould will suffice. Thats again just as it sounds, take a wax paper cup, cut the bottom out, flip it upside down, hot glue it to the base with the bolt centered inside the upturned cup (coat the interior of the cup with mould release or a thin coat of Vaseline). Slowly pour silicone in the cup using a gloved finger to spread it over the threads. One trick when moulding items with threads is to polish them. This way silicone flows over the surface with less resistance. It also allows air bubbles and pockets to escape much easier.
When the silicone is cured, unscrew the mould/bolt as one piece from the base, remove the wax paper cup, use a new x-acto blade to slice the mould open on one side in a strait line, not hacking, slice, as clean as possible. The bolt should come out easily. If not some compressed air from an actual compressor will pop it out. Dont use canned air, just, dont. Doesnt have the pressure needed plus it also leaves residue inside the mould. To seal the mould back up to pour parts, simply place it back inside another wax paper cup! If youve sliced it nicely, that seam will vanish when placed back into a new cup. Repeat the master bolt removal when pulling the now cast part.
One trick that should help greatly with casting parts like this is graphite powder. Its used as lubrication for machinery. But if you brush it into the mould, air bubbles have absolutely nowhere to hide and grasp onto. The parts will also come out metallic in finish if your using black casting material. I highly suggest other material than cheap casting resin. Ive made nuts and bolts out of high impact plastic and used those cast parts to assemble the items they were cast for. No breakage, highly durable yet also expensive, about $100 a gallon.
Ive done one piece nut moulds with no issues. Typically only one side of the nut shows so the other side can be whatever as long as its to not show.