Try methylene chloride, it will perform a cold welding together of the pvc by a slight disolving and then drying out. The two sides must be very clean - like brand new. Otherwise, try PVC piping cleaner and glue - they work great and are used for water piping. You should be able to find a bulk supplier. This assumes you have gotten no support from your PVC sheet supplier - go up the ladder of distribution and get to the manufacturer of the sheet, too.