You shape the rubber around a favourite item to have a long-lasting candle mould of that exact shape with no seams. Terra cotta pots of all shapes and sizes are charming filled with a candle. You seal up the hole in the bottom by lining the inside with a little aluminium foil, and the outside with sealing putty. You can paint the outside of the pot with craft spray paints, or use stencils. Other crafters have found it helpful to seal the pot with non-flammable varnish so the wax and colour doesn't seep into the terra cotta.
Galvanized buckets come in nice small shapes, just perfect for candle making as well.
Try making citronella candles left in the buckets for outdoor use. Citronella candle scent is available from candle making or craft supply stores. Make some 'industrial' or very rustic looking candles using corrugated cardboard. Make a mould from a sheet of cardboard in a cylindrical shape open at both ends for a pillar candle. Set the mould on a paper plate lined with foil.