I designed this mold for the "Build A Castle" challenge. My goal was to create a mold that would print upright and look about like the sand castle it creates. I also didn't want to have to rely on the slicer to generate support material.

I designed all the vertical edges with a slight taper to help with release. I also designed in some easily removable supports to help the top print better. It took a few revisions to get the supports right and multiple versions are included. The R3 version prints the best, and it wastes only about 4g of PLA for support material.

There aren't any beaches in the middle of these corn fields, so I had to do my testing in a bucket. The mold works as intended as long as it gets packed tight enough. Make sure you get enough sand packed into the spire before filling the rest. To release just squeeze the sides a little, then tap around the spire and other faces before gently lifting off the mold.

Design files here in multiple formats. Select your format and use the download button at the top right.

The R3 version prints with no extra support. I recommend using PLA because this design is prone to warping. Use a brim if your printer has room (print is about 148mm square). Included support material will need to be cut away from base, but the tops should break away easily.

My (slic3r) settings:
.4mm width
.2mm layer height is slow, but turned out better
.3mm layer height printed much faster, but corners started to lift
2 shells
3 top/bottom layers
5% infill with solid fill threshold at 0

Make sure you aren't having any bed adhesion problems, this print will put that to the test. The corners will pull off the bed if you're not careful, but don't worry too much if they do lift a little. The support webbing in the middle does a pretty good job of keeping everything from moving around. You can see the attached picture of the finished print with lifted corners.

I didn't have trouble with the corners in my first revision like I did in the later ones, but I'm pretty sure that is related to increasing the print speed and layer height.

Also make sure your bridging is working properly as there are roughly 30mm spans to bridge.