Seems like their is an abundance of these tutorials on here, but I'm really happy with my trailer so I'm going to share it anyways. This isnt going to be to detailed, more of an overview that people can draw inspiration from or maybe make suggestions I hadn't thought of. I'll list materials used as I go through the steps. I tried to make as much of this as possible for either free or cheap, particularly from materials salvaged/found.
Step 1: Starting Out..
I bought a trek transit trailer from the neighbor for $20, I heard these are now manufactured by chariot. Pic is not mine, but pretty sure exact same. Using a boxcutter and screwdriver and some wrenches, I cut off all the nylon and took off all parts except for frame and wheels. I threw away nylon but kept all metal pieces, buckles, straps etc...
Step 2: The Body
For the body I decided to use some wood I found from IKEA bed platform someone threw away and an aluminum speed limit sign I found, which like the frame was aluminum and therefore lightweight and resistant to rusting.. I chose to make body the way I did because of pre existing holes and a metal protrusion where wheel dropouts were that Seemed like would be good added support. The slats were warped/bowed upwards, which turned out to be a good thing. I used drill press to make a few additional holes in frame and hand drill for all other holes. Most every hole on trailer was conveniently 1/4". I used bolts to attach slats to frame and pop rivets to attach street sign platform. When drilling and attaching holes for slats, I clamped them to a straight piece of wood and removed after attachment so that the bowing of the slats built into body added tension. The street sign was coincidentally a very near fit. I decided to make this trailer with a flatbed type design, using a system of holes, hooks, bungee cords, rope etc to attach whatever cargo or cargo containers necessary. I liked the open ended aspect of this as opposed to a fixed basket/box type design. I split the difference of street sign that needed to be trimmed equally on each side and this made a nice lip just big enough to drill cord/bungee attachment holes. There was a U shaped tube that was at front end of trailer I flipped the other way which made a good stopper bar at front end. I made sure to keep this bar in place when attaching slats and bed to help frame keep form.
Step 3: The Hitch
The trailer came with the original arm that connects to bike, but no hardware to attach it to on the bike side of things. The replacement kits were a little expensive, so I poked around some boxes of stuff until I found a solution. First I found an old shower/faucet piece that had a ball and socket type joint, perfect for full range of motion that could be loosened or tightened depending on how easy you want it to move. I took the head off of itand screwed it onto a piece of plumbing pipe. Dont remember what size, but its fairly common, I think 1/2". It was just barely to big enough to fit inside of square aluminum tubing because of some small ridges in tubing, so I shaved it down with an angle grinder into a slightly squarish shape until it fit snugly into the frame tubing far enough down I could drill a hole and bolt it through the original hitch piece hole. This fit nicely inside of an aluminum hitch made by burley. This is one of the few things I bought new, and the $20 seemed worth it compared to a day of messing around with other ideas. I like it so much I recommend it and plan on using them again if I make another trailer. Hurley forged hitch. I bought a ball locking cotter pin from home depot for a few bucks to go into it. The shower head hitch is the only possible weak point I'm worried about because it is plated brass, however it is very thick and has so far held up fine over many long rides.
Step 4: Wrap It Up
A few final touches. I ordered some mags off ebay, because they are bad ass and remind me of my BMX days, and the axle seemed thicker than the one on the spoked rims. There is a milk carton attached with some zip ties in some of the pics. I took a piece that I removed from the beginning and put it on the front of the frame to act as a kickstand. Not sure what it was originally. Overall, it works very nicely. Not sure what the weight limit is, but feels pretty sturdy while remaining lightweight. Pulls very well, and I try to keep the weight towards the back since the wheels are not quite centered, but this works great as weight is shifted back with forward momentum. Oh yeah, led strip on bottom, gotta have LED's ;) Hope this helps or inspires some folks interested in building a trailer. Salvage things that look useful whenever you can, even if you dont have a purpose for it at the moment, it may be the exact thing you need at some point in the future.
*I love the simple aesthetics of my bicycle, but have found it extremely difficult at times to keep upright when standing alone while attached to trailer. I dont want to add a permanent bulky kickstand, and have found maybe 2 options in the retail biking world for removable kickstands that are pretty pricey, so I think my next instructable will be a DIY detachable, compact kickstand, as soon as I figure it out ;)