I used PU40 from Toolstation about 5 years ago when I replaced my keel bolt with stainless - PU40 is a cheap version of Sikaflex but seems to work just as well. I'm told that CT1 works as well.
The trick is to apply a good layer beneath the plates and partially tighten the nut by hand until the sealant just oozes out evenly. Then let it cure overnight and tighten the nut to around 18 ft lbs and no more, or you risk crushing the glass box. If you tighten the nut before the sealant cures it will all ooze out and may well leak later on. Mine has been fine for 5 years cruising about in Lyme Bay and on the Camel estuary.