ESI Front Rail Seal 6ft EPDM Rubber Fills a height gap of .251" to .487" from the sidewalls to the front rail when mounting a truck cap or camper shell If height of the gap is .250" or less, just install one ESI high quality EPDM rubber truck cap seal on all 3 rails (see product description) If height of the gap is extra large, from .488" to .937" install Front Rail Seal XL (ASIN: B07147Y43J) Preferred by professional installers over PVC/foam seals No need to replace this seal every time your cap is removed.