I carry my plastic kayaks upside down on the roofrack of my minivan.

To make sure it sits tight and secure I attach a pair of 2 x 4's across the rack; they have deep grooves running across them that fit the cockpit coaming so the boat sits snugly. Then I place straps over the whole thing so they hold the 2 x 4's down as well as the boat - saves me making fancy clamps for the 2 x 4's. One side of the 2 x 4's is cut to fit the short kayak and the other side is cut to fit the long one so I can have either or both on board.

The straps run over a boat down to a heavy-duty eyelet at the center of the 2 x 4 and back up over the second boat if necessary, so the pressure on the boats is uniform on both sides. I also run a safety line from each boat to eyelets I fitted to the front of the van; this acts as a tell-tale in case a boat should move and, if it should become loose, it will keep the boat attached to the vehicle. I have a red rag on the back of the boat as well, so I can spot movement at either end with a quick glance from the driver's seat.

It's probably over the top but it adds to my feeling of security and I can concentrate on my driving. Put up/take down time is about 10 minutes including the 2 x 4's, although I can leave those on if I want.

For the canoe I don't need the 2 x 4's, I put foam blocks over the gunnels and lay it directly on the rack with the straps over the top which saves time. That is OK for a short trip but it's not secure enough for a longer trip so then I use the 2 x 4's.

The canoe is very light - a one-handed lift - but the kayaks are much heavier and it's hard to get a grip on them. As I aged I began to have difficulty lifting them. I drilled a 1" hole in the end of one of the 2 x 4's which accepts a short aluminum tube. This sticks out over the side of the van while loading/unloading the boats, so I can stand the boat on end and lean it against the tube, then I lift the other end up. The tube supports half the boat's weight and helps a lot.