1. Keep the rebar position mostly close to the surfaces (top and bottom). About 1.50-2.00 cm clear space is recommended.
2. At every corners, extra diagonal bars should be added both top and bottom.
3. Take care the concrete placement, good vibration (not too more or too less energy and at an appropriate time), avoiding bubble under large padding areas.
4. If possible, pls discuss with concrete supplier for a mixture with less bubble. Normally, concrete admixture create bubbles to facilitate concrete flow.
All the comments above are to avoid any aesthetic cracks. You should call for a meeting for every parties and criticize about the quality goal. Cracks will be common for general cases, but it is not for this project.