Assuming that there is SOMETHING there that you work from.
You can be as crude as to simply bridge the span with masking tape and lay up directly on the masking tape. If the span is too great and the masking tape alone won't support the first layer, you can use cardboard as a makeshift mold. It is easier to release the cardboard if you cover the side, in contact with the resin, with masking tape.
Once the first layer is cured and attached, that becomes the structure that you build your other laminations on.
You may have to do some finish work where your makeshift mold was.