I've never used any sort of official "J racks," but used to tie four or more kayaks onto roof racks made of padded boards (1 x 4 or 2 x 4), and in that case we just fastened a pipe base to each board, then screwed a pipe upright into it (that could be removed when carrying fewer kayaks, only leaving the base). We then leaned the first kayak up against the upright (which could be padded with a modern pool noodle or pipe insulation), and subsequent kayaks up against the first one, and then tied them on. Kayaks are (or at least were) strong in that direction, so they were not damaged by not having special cradles.
Of course one reason we didn't have J cradles then is that no-one had invented them, but also, it's a lot easier to store a 2' length of pipe than a J cradle, when not using it (we could just keep them in the car). I can see where the cradles could be nice, especially for a long/heavy/sea kayak (but then one is not likely to have a boat like that on a C-Dory).
I would also wonder if the J cradles shown would rust annoyingly in a salt water environment. Lots of crevices, etc.
Of course one reason we didn't have J cradles then is that no-one had invented them, but also, it's a lot easier to store a 2' length of pipe than a J cradle, when not using it (we could just keep them in the car). I can see where the cradles could be nice, especially for a long/heavy/sea kayak (but then one is not likely to have a boat like that on a C-Dory).
I would also wonder if the J cradles shown would rust annoyingly in a salt water environment. Lots of crevices, etc.