An adjustable circumference seal, comprising: an envelope having an outer layer and an inner layer, the inner layer defining a sealing surface; a circumferential closure for constricting the inner layer of the envelope and thereby adjusting a circumference of the inner layer of the envelope, the closure comprising: at least one cord channel in the envelope; a cord threaded from a cord hole in the outer layer of the envelope through the at least one cord channel, such that, by pulling the cord, the circumference of the inner layer of the envelope is reduced until the sealing surface is brought into a sealing engagement; and at least one fastener for maintaining the cord in a selected position, characterized in that the circumferential closure comprises a band inside the cord channel, the band having a first end and a second end, with a first cord guide at the first end and a second cord guide at the second end; wherein the first cord guide is a first loop and the second cord guide is a second loop, and in that the cord travels along the cord channel from the cord hole to the first end of a band, where it passes through the first loop arranged therein, then back in the opposite direction from said first cord guide to the second end of the band where it passes through the second loop arranged therein, and then reverses direction again and travels along the cord channel to pass through the outer layer via the cord hole for reducing the circumference of the inner layer of the envelope until the sealing surface is brought into a sealing engagement when said cord is pulled. an envelope having an outer layer and an inner layer, the inner layer defining a sealing surface; a circumferential closure for constricting the inner layer of the envelope and thereby adjusting a circumference of the inner layer of the envelope, the closure comprising: at least one cord channel in the envelope; a cord threaded from a cord hole in the outer layer of the envelope through the at least one cord channel, such that, by pulling the cord, the circumference of the inner layer of the envelope is reduced until the sealing surface is brought into a sealing engagement; and at least one fastener for maintaining the cord in a selected position, characterized in that the circumferential closure comprises a band inside the cord channel, the band having a first end and a second end, with a first cord guide at the first end and a second cord guide at the second end; wherein the first cord guide is a first loop and the second cord guide is a second loop, and in that the cord travels along the cord channel from the cord hole to the first end of a band, where it passes through the first loop arranged therein, then back in the opposite direction from said first cord guide to the second end of the band where it passes through the second loop arranged therein, and then reverses direction again and travels along the cord channel to pass through the outer layer via the cord hole for reducing the circumference of the inner layer of the envelope until the sealing surface is brought into a sealing engagement when said cord is pulled. at least one cord channel in the envelope; a cord threaded from a cord hole in the outer layer of the envelope through the at least one cord channel, such that, by pulling the cord, the circumference of the inner layer of the envelope is reduced until the sealing surface is brought into a sealing engagement; and at least one fastener for maintaining the cord in a selected position, characterized in that the circumferential closure comprises a band inside the cord channel, the band having a first end and a second end, with a first cord guide at the first end and a second cord guide at the second end; wherein the first cord guide is a first loop and the second cord guide is a second loop, and in that the cord travels along the cord channel from the cord hole to the first end of a band, where it passes through the first loop arranged therein, then back in the opposite direction from said first cord guide to the second end of the band where it passes through the second loop arranged therein, and then reverses direction again and travels along the cord channel to pass through the outer layer via the cord hole for reducing the circumference of the inner layer of the envelope until the sealing surface is brought into a sealing engagement when said cord is pulled.