Some small tips:
If you are going to use fibreglass you don't need concrete mix, just sand and cement. So it makes it a lot easier to work...
If you add a fine bead of silicone to your internal corners in your form you will get chamfered edges on your finished piece and no seepage from the mould.
You can lightly sand the finished product if it has any high spots, without aggregate you won't get that ground concrete look, but care is required to not expose the fibres.