Replicape case in OpenSCAD
No grill is provided for the fan. It mounts to the outside of the case, so there is no use in having an integrated grill on the inside. Many grill designs are available for fans, printable and manufactured, and the user must select one with appropriate safety for their fan and situation.
The Beaglebone with Replicape should snap into the case, and the case should snap together. Press up on the bottom of the case while inserting the Beaglebone to slightly spread the clips when inserting or removing the Beaglebone. The case clip legs are a bit fragile, so take care inserting them. Insert both clips just inside the edge of one side, then gently pinch the legs on the other side into the case, and slide together until it clicks.
For slicing, look at the example parameters at the end of the replicase.gcode file. In particular, print at least the layers with text labels at .15mm or thinner if you care that they are legible.
The most likely parameters to want to change are the fan parameters at the top. The offset is the X and Y offset from the center of the fan. The width of the mounting flange and the other screw diameters are perhaps the next most likely. The rest of the parameters are mainly for model development, and changing them may have odd or unintended effects; the model was developed as a quick transcription of measured and documented sizes and is substantially over-constrained, so it is easy to specify nonsense.
Use M3 x 6mm-8mm or #4-40 x 1/4" to mount the beaglebone in the bottom if you are not satisfied with the clips alone. Depending on your printer, you will probably have to drill those holes out to an appropriate size.
Originally inspired by https://www.thingiverse.com/thing:2610261 but no code or shape is preserved in this model.