I have increased the number of kayak I carry in my truck bed so now they need to be vertical in J Cradles.
Don’t ask me why I just don’t buy the off the shelf carrier solutions…I’m a tinkerer… 
I currently fit 2 kayaks in my truck bed. I have a bar extension that would support the kayaks at the farthest end. I would like to carry 4 kayaks now. This will require to fasten them on their side. I now need J cradles to fasten the kayaks vertically and fit 4 kayaks this way.
So, I was thinking about taking a standard plywood sheet to which I would attach two loops that would allow me to fasten the plywood sheet to the tie downs in the truck bed. Onto the plywood sheet I would permanently attach J cradles with screws.
Whenever I would need to transport kayaks I would attach my extension, place the plywood sheet in the truck bed, fasten it to the tie downs and then load the 4 kayaks onto the J cradles.
Which brings me to the main question that I hope someone knows the answer to. Where can I buy padded J cradles that have screw holes in the bottom?
German ones
I have some German ones that come from a company in Nova Scotia owned by Bob Thorne.
I would drop that for a bad idea before you start.
If you have a good base under the plywood just secure an upright and lash the boats to that.
The whole program sounds sort of chinsy to me but you may have something in mind that I don’t know about.
Thinking about your project::
A sheet of 3/4" plywood with a bunch of J cradles will be way to heavy to put in and out of the truck bed by yourself.
And I would think that if you used 5/8" plywood there wouldn’t be enough thickness of the wood to keep the screws from pulling out when you slide each kayak in and out, unless you bolt the J cradles in and then the bolt heads or nuts will ending up scratching the bed of the pick up when you slide the plywood sheet in.
Why not just get two “Cargo retaining bars” and attach the J cradles to them. You could put a couple of blocks of 2"x4"'s under the ends of each one to keep them from slipping down to the truck bed from the weight of the yaks, or just mount them at the bed level. - should be a easy in and out installation
Cheers,
JackL
low loading angle
I need to have a low loading angle for the kayaks. Putting then high on the bars is not an option at this time. I do agree that the plywood is not an elegant idea, but how do I attach J-Cradles to the truck bed without really damaging the truck bed?
Stackers
Seems like all you are trying to do is duplicate a Yakima Stacker.
http://www.yakima.com/racks/kayak-rack/product/8004036/kayak-stacker.aspx
PVC pipe and pool noodles are all you need.
Evidently you don’t understand…
what a "cargo retaining bar" is:
It is a two piece adjustable length bar with small metal plates on each end that have rubber pads on them. It is threaded.
You place it in the truck with the end plates against the side walls of the truck, at any height you want, (in your case it would be down near the bottom of the bed) and then rotate one of the bars until both ends are snuged up against the side walls. I have tightened mine so tight that I could stand on them.
You would need two of them. attach your J cradles to the bars and mount one of them across Just in front of the rear wheel fenders, and the other back across just in front of the tail gate.
the reason I said put a couple of 2"x4" blocks under them is if you have four plastic boats on top of them, the weight just might push them down, and this will keep them just off your bed.
-EaSY IN AND EASY OUT with just a few twists and each one would probably be only about 15 pounds
Now I fully expect to see Yakama come out with this set up in the coming year called "truck bed J cradle holders"
Cheers,
JackL
Make your own?
http://outdoors.webshots.com/photo/1104181362040906646PqXQgQ
http://outdoors.webshots.com/photo/1112327361040906646GlMxgt
http://outdoors.webshots.com/photo/1112327704040906646CItTxx
http://outdoors.webshots.com/photo/1112328281040906646tUjctQ
http://outdoors.webshots.com/photo/1112327931040906646QCxJPo
yay
You are right. I can just take pvc pipes with elbows and construct a skeleton with two rows of vertical spikes padded with pool spongies (as you suggested.) This will work out just fine and for very little money.