In the absence of a well-made LDPE, PLA, or polypropylene mattress cover, some folks go the DIY route and make their own mattress cover using LDPE sheeting. This is often available at very low cost at hardware stores. Be sure to find sheets that are at least 6 mm thick, however, to prevent off-gassing. And be very diligent about taping or tucking seams to seal in VOCs.