http://www.botach.com/products/kriss-glock-20-21-17rd-magazine-extension-kit.html
It will need a custom mag spring to be made. The Kriss extension spring is for the .45 ACP mag and is wound larger than the 10mm spring. It won't fit inside the Glock 20 mag because they molded the inside smaller than the 21 mag. The outside dimensions are the same and will assemble and fit into the Glock 20, it just won't accept the Kriss spring.
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