I started with an old trailer that was given to me by a "friend". First mistake
Disclaimer: I am not an engineer or pretend to be one. This is just guide of the method that I used to build my trailer. Please check your local laws before attempting to build your own. I was a pipefitter all my life and did quite a bit of welding. Do not attempt to weld your own trailer unless your qualified to do so. By all means please work carefully and use all the necessary safety equipment required. After 60+ plus years I can still count to 10 on my fingers! Can You?
As I took possession of my new trailer I knew I would have some work cut out for me. The crossmembers were in good shape, new 13" tires, new bearings and hubs. The problem was the side rails were very rusty, the trailer was too narrow for my paddles to fit between the tires. If I was to use this trailer a lot of work would be required to put it in useable condition.
I decided that it might be better to just "bite the bullet" and go and buy a new one. After much research and looking at many trailers I found problems with all of them. My paddles are 83 inches to the outside edges and most trailers are have 82 to 84 inches between the fenders. A couple that were 86 inches had drawbacks in the axle area for mounting it forward enough to give me a reasonable tongue weight. The main drawback was the $$$$. I really didn't want to spend close to 3 grand just to get to the lake with my new toy. It was then I decided to take another look at that rusty old hunk that was in my yard,
The old trailer frame was just 72 inches wide and the crossmembers after cutting apart wound up at 66 inches. I added 7 inches to eachside of the crossmmber for a width of 80 inches plus the two sides making it an even 7 foot. The fenders are welded to the outside rails giving an extra 1 1/2 on each side. That allows the paddles to fit right between the fenders and the overall width came out at 8 foot 5-1/2 inches a full 1/2 under the limit.
The steel for the sides was 24 foot long so I needed to cut 3 foot off each one of them to give me the 21 foot needed for the sides. A pie like cut was made 10 foot from the back on the inside of the rails in order to bend them in at the front. A crossmember was installed at this point and an extra support was welded in to give the side joint support. It was all built on 3 folding tables that was leveled very carefully so the trailer would be square and warp free. After welding in all the crossmembers the old hitch was rewelded on the trailer. Its starting to take shape now. On the rear is a non-adjustable bracket for the keel bunk
. The rest of the brackets will adjustable to fit the bottom of the boat as well as to allow for some flexing that will be in the trailer. It took me 5 days to get this far. I lost a day or so just looking at other trailers to steal my ideas from. While it was still up on the tables I decided to give it a coat of paint as the new steel was already starting to rust. The trailer was then turned right side up and supports added for the diamond stepplates. I would need to put it under my boat and find the balance point so I would be able to install the axle. 
The boiler, engine and paddle wheels were installed in the boat to find the C/G in order to weld the axle on the trailer. 
I used the engine crane to flip it over. When I set the axle in place I needed to be 5" back from 5th crossmember. Before I welded it up I checked the axle to see if it was square with the side frames. I just set the axle by measuring from the rear of the trailer. To check I also measured from the center of the ball hitch to the outside of the hubs. Everything seemed fine so I welded her up. I would have been pretty mad had I built myself a trailer that went down the road "doglegged". 
With the axle now welded into place the bottom was given a second coat of paint and then turned right side up. The chine bunk supports were welded in place. The fenders were salvaged from the old trailer and rewelded on to the new one. It was given a final coat of paint and the lights were added. Because of the 8'6" width it was necessary to add 3 marker lights to the rear cross member. I also installed an amber side light on each side of the trailer. Time is now up to about 15 days with a lot of coffee breaks included. A have quite a few retired friends that stop by often to give me mainly moral support. (I really didn't think that my morals were that bad)
With the boat now on the trailer I decided to see how it liked being pulled around with my old van. The van being a 4 cylinder I knew it would be difficult to haul the completed boat and trailer to and from the lake. To my surprise the rig towed quite nicely. A little planning before coming to large hills was needed as the 4-banger in the van labored to make it over some of the larger hills in my area. The view looking out my "stock" side mirrors was quite adequate. I had visions of big ugly strap on mirrors because of the wide width of the trailer. In fact the boat sits low enough on the trailer for me to use the inside mirror. When going around corners one has to be careful as the trailer "tracks" about 2 feet to the right and left when making 90 turns. Even my small utility trailer does this to some degree. 