Monty Sailors, If, for some reason you must alter the factory slot on your mast the aluminum is amazingly maleable. I was going to buy a very expensive slide door but, just for grins (?) decided to close down the length of the slot myself so my gooseneck would not slip out of the mast. First in inserted a piece of oak that was a tight fit into the mast slide such that the bottom of the oak extended as far into the slide opening as I wanted to close it down. Then I took a second block of oak and tapped it with a hammer carefully and ,voila, the slot closed up very nicely. My result was that no matter how much I downhauled on my new mainsail, the boom now stays in the slide tunnel on the aft of the mast. Quick, cheap and fun. Fair winds, Tom B, Monty 17 AS-IS, now splashing around in east central Texas. <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=icon> Virus-free. www.avast.com <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail&utm_term=link> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>