What I use is the following:

Slot gaskets used to be my worst nightmare. Nothing I did would last through the season. Tried various weights of mylar, plus the special slot gasket material sold through various chandlerys.

The trick is to make your own. I use medium weight mylar. But the trick is to adhere sticky back sailcloth to BOTH SIDES. Contact cement doesn't like mylar, so why try to force it? When you try this, the trick is to pin down the sticky with the smooth back to a board, sticky side up. Then you roll the mylar into a tight roll, affix one end, and carefully roll it out onto the sticky cloth, doing one side at a time. THIS IS A GAME OF PATIENCE. Don't worry about bubbles. If you get bubbles, you stick them with a needle and force the air out. Thank you Phil for recognizing THE FLAP and the notches. For all who don't believe......well, too bad. The last time I did gaskets was for the 1992 Worlds....

Ok you silly people! They make slot gasket material, fuzzy on one side and smooth on the other. You buy it by the foot and can get it at layline or APS. If you put it on properly it will last forever. Do yourself a favor and buy this stuff otherwise you will keep fighting and spending time playing with slot gaskets instead of sailing!

I have no problem getting the slot gasket to stay attached to the hull, but I do have a problem keeping it straight. I've used Mylar, Mylar with sailcloth, and just plain heavy sailcloth. They all sooner or later, usually about the 2nd or third time out, stick on the board when it is being raised and get partially pulled up into the slot, which then puts a crease in them and from then on part or all of the slot gasket gets drawn up into the slot with the board. My other problem is that my board hangs down below the slot at the aft end when it is on my dolly. This puts a permanent curl in the edge of the gasket and it never closes properly when the board is all the way down. I know I should devise some system to keep the board fully elevate when on the dolly, but haven't yet. Do others have these problems? I also have an alternative to the "roll-up" method proposed by Crump for attaching anything with contact cement. I first apply the contact cement to both the hull and the slot gasket and let them dry. I place the slot gasket on wax paper that I have laid out on the bottom panel beside the c/b slot. When the cement is dry and I am ready to attach it, I roll out a length of wax paper to cover the entire cemented slot and then place the gasket on to the wax paper, glue side down, so that only the wax paper prevents permanent contact. I then align the gasket how I want it and when satisfied, I pull the wax paper out one end until I can just make contact at one end of the gasket. Once one end is attached, you simply pull the wax paper out the other end and press the two pieces together as you go. This works for all contact cement applications.

Various options exist, most of which will be posted here on this thread, I am sure. I use heavy weight mylar (the type used in some drafting offices) glued on to the hull with "good" quality contact cement (making sure the hull is absolutely clean prior to applying the glue), and then I cover the mylar with dacron sticki-back sail repair tape. Don't use the spinnaker repair tape, it is not heavy enough, use the stuff that is used to repair headsails, etc. When you glue the mylar on, it should go past the forward and aft ends of the C/B trunk by about 6", and extend out from the sides of the trunk by at least 2". Don't glue the mylar right to the edge of the trunk, leave about 0.25" without glue to allow the mylar to open up without pulling off when the board comes through. When putting on the sticki-back, overlap the mylar by about 0.25" to 0.50" to protect the edges from lifting.