For my concrete object I chose to make a vessel for cooling metal. In the past I have used glasses I don't really care about or pickle jars, but the shape does not allow for certain objects to fit nicely, and I was always scared of the glass shattering due to rapid heating. Having the wide open face solves my size issue and concrete should be more forgiving, as cracking is a lot more tolerable than shattering.
I made my mold using two parts created from pink foam insulation: a box with no top to get the general shape, and a rectangular prism as an insert to create the channel. The sides are not totally square, but for its use I was satisfied with the mold.
Casting the object was not too difficult. I mixed the concrete with my hands and first poured a layer of aggregate for the bottom. Then I put the insert in and mixed more concrete, this time hydrating it more and adding topping mix. Then I poured that mixture into the mold and banged it on the table to level it.
There are a couple things I would have changed had I done this again with my current knowledge. First, and most importantly, I would have made the channel deeper. As I did not secure the insert in anyway, I believe the pressure from the concrete pushed it up, resulting in a shallow impression. I wanted it to be at least an inch deep, but it only ended up around half an inch at the shallowest point. Second, I wish I spent more time designing my mold to be more interesting. While the purpose of my object is purely functional, it would have been nicer if it wasn't as ugly. Perhaps even just dying it would have made a big difference.
The mold survived way better than I thought it would, and the insert was actually easier to separate than the box. I was anticipating needing to destroy the insert since I didn't use mold release, but it actually just popped out.