Surface sterilize them with 3% hydrogen peroxide and then leave them in sunlight to break down the peroxide. I would make up 100 mL of 3% hydrogen peroxide with a drop of tween-20 (or dishwashing soap). Use spray bottle to spray all the surfaces. Do this in your laminar flow hood (or whatever sterile space you are using for sowing seed). Put the lids on the tubs and then place these in a large plastic bag. Spray the inside of the bag thoroughly to make sure everything is sterile and then seal and place these in sunlight. The thin polypropylene naturally breathes so there is no need to vent them.
I used this method myself and had excellent results. I had tubs remain sterile for years (and were thrown out in the trash sterile when I finally gave up on the seed or age dried out).