The idea came for this the first time I saw one of the "tag along" bikes for kids. I liked the idea of having a trailer that tracks with the bike. I ride on public roads and with a two wheeled trailer there is always concern about making sure of the position of the trailer on the road. This is just over two feet wide and tracks directly with the bike, eliminating that concern.
I like to make use of things I have and don't really like buying a lot of stuff to complete a project. The majority of what I used was stuff on hand. I also build things off of ideas, I like the process of designing and solving how to put something together. This instructable shows the major steps I took.

Step 1:

I have been watching Craigslist for a tag along for a while. Most of the ones listed were around $40, reasonable enough considering new ones are more than twice that. I found one in a thrift shop for $15. It was missing the hitch so I had to build one. I included that in the instructable but if the unit you use has one great.
First step is obvious, strip off unwanted items - handlebars, seat and pedels. Simple hand tools will do the trick for all of it.

Step 2:

The standard hitch that comes with these is far superior to the one I built. I did find just hitches but they were around $30 so I figured I could make my own. Obviously if you have a hitch you can skip this step!
The biggest issue is keeping the trailer upright and not letting it flop side to side.
I used a piece of tubing as the intermediate piece. I had to notch it at the front so it could pivot side to side.
The bracket off the seat post is from a fixed wheel caster without the wheel on it. I used a single u-bolt to attach it to the seat post - using 2 u-bolts would have eliminated some slop that I encountered later.
The bolts have to fit very snug in the holes. I used a shank style bolt with partial threads just on the end and a solid shaft to aid in snugness of the fit. Initially the bolts and pull pin I used were considerably smaller than the hole and there was unacceptable movement side to side.

Step 3:

One of the goals of this project was to be able to haul more than could be put on a bike. I went through several ideas of what to use for the storage. I thought standard bins would be on the wide side so I looked at several stores to find the size I thought would work. These are 7.5 gallon Sterlite bins I found at Wal-mart for $6.50 each. They are about 12 inches wide which is narrower than standard bins, perfect for this project.

Step 4:

Once again I like to use what I have on hand.
The front section actually has an angle on the front side. The angle gives the piece strength, using flat stock may not offer enough support and may end up bending. I attached it to the frame using the original kickstand hole.
The back brackets were flat sections from a weight bench. I bent them and attached them to the axle bolts.

Step 5:

I clamped the end of the tag along to a saw horse while I worked on it to keep it in place and in about the position it would be on a bike. This helped when mocking up the position of the tubs. In these pictures the tubs are just setting on the brackets so I could see if they were in the position I wanted.

Step 6:

After locating the bins, I just drilled holes and bolted it in place. Note the extra large washers - these are used to keep the bolt head from pulling through the plastic. I did this with all areas that have bolts or screws through the plastic.

Step 7:

I was originally going to make a pvc frame to go around the top. Once I got the bins bolted down I felt like it didn't need a full frame, just something to hold them together.
I used a couple of pieces of wood to locate them, one across the front and one towards the rear of the bins. This made them feel and move as if they were one unit. The flexing when loaded will be distributed across this and help to keep them from pulling on the lower mounting brackets. I used some wood screws and again used wide washers to help keep the screws from pulling through the plastic.

Step 8:

Onced hooked to the bike I realized that empty the tag along was unstable stationary and would be worse once the bins were loaded. I found a kickstand in my stash and used the bolt on the front corner that was holding the bin to the bracket to attach it. Problem solved. This kickstand will actually support the entire rig.

Step 9:

Pictures of the finished product.
I may change the wood to metal at some point.
It is March in Michigan so other than a quick test drive I haven't actually hauled anything in them. It tracks very well and there is more than enough storage in the bins for me. Can't wait for summer to use it!