Kayakers tend to collect boats and those who take up the sport pretty soon find themselves needing a canoe trailer to carry them all. If you already have a trailer, there's a simple conversion you can do that is removable, easy to build and an inexpensive solution to your kayak hauling problems. All you need is basic carpentry skills and some power tools.
Measure, mark with a speed square and cut four 2 inch by 4 inch uprights into 8 foot lengths. Cut eight 2 inch by 4 inch boards the width of the inside floor of your trailer. Cut four 2 inch by 4 inch boards 6 feet long for the lengthwise members.
Measure the length of the inside of the trailer. Lay two of the uprights flat and screw two of the 2 inch by 4 inch long beams to the tops and bottoms of the uprights. Make a second identical assembly with the other uprights and two long beams. Screw at least two screws at each connection point.
Turn the side assemblies so that the long beams are on the inside and set the side assemblies inside the trailer on either side. Screw four cross-members on the ends starting with the first one even with the top. Screw the remaining three cross-members 2 feet apart going down the end of the frame with the top edge of the last cross member 2 feet above the bottom of the side assemblies. This creates four crossbeams at each end 2 feet apart on a 6-foot long frame. Make sure everything fits inside the trailer bed.
Wrap carpet strips over the tops of the cross-members to pad the frame where the kayaks will sit on top and tighten them in place with zip ties.
Attach the long 1 inch by 2 inch braces with 3 inch screws to the side assembly on the outside of the frame. Run the brace from the lower part of the front upright co the upper part of the back upright to provide an angular cross brace.
Slide the kayaks in over the back cross-member and set the front end over the front cross member. Tie the boats in with heavy bungee cords. If your trailer is wide enough you can set up to eight kayaks on the frame with room below for paddles, lifejackets and gear. Tie the frame down to the trailer with ratcheting tied down straps.
Use pressure-treated lumber and you won't need to paint the frame. When not in use, lift the frame out of the back of the trailer. Set the legs on concrete pads or blocks to help prevent the wood from absorbing water from the wet ground and rotting.