Step 1: To Be Improved...

The hitch: too long, introduces vibrations...

The axles: the handcart wheels wobble a lot

The distance between front and rear (too short)...

Not very usable: constant tuning, breakdowns, lots of wear and tear...

Step 2: Improving the Hitch

A new hitch was made, to be connected to a receiver connected to the rear wheel axle, instead of connecting to the seat post, by means of a small swivel wheel receiver. I got the idea from https://www.instructables.com/id/Bicycle-trailer-hitch-and-fishing-rod-holder-combo/
The receiver wheel is quite small: if it will show signs of too much wear and tear, a larger size needs to be selected as replacement...

Step 3: Improving the Axles

What is wrong with this picture???

The wheels have play horizontally.

The wheels can wobble away from a perpendicular position towards the axle.

Remedy:

1: Shorten the bushing (wheel size).

2: Apply lots of grease.

3 (Perhaps later: turn new, cylindrical bushings).

Update: replaced front wheels with better, non wobbling ones: the newer handcarts of the same model use better bearings. Someone needed more space in his shed, and sold his handcart for E 4.- !:)

Step 4: Improving the Standard Size

Now, long posts of light ply material connect the 2 riding parts. They can be interchanged by posts of any size, if needed. For now, the length is ca. 2.50 m ( 10'), enough to carry full size sheets of plywood.

As long as traffic is not hindered and protruding loads are marked with a bright flag, everything seems to be legal.

Wide loads (> 1m) are not really legal. But if towed on a quiet Sunday morning, it would not create many problems. Most cops really like the looks of the trailer and don't know obscure laws about them.

There is no limit on length.

Step 5: Conclusions

1 A 4 wheel trailer is a rather difficult project. Looking at existing trailers can only tell a part of what one needs to know. Downscaling the design of big truck trailers was a great idea. But the materials used are different, sometimes imperfect; that's why some unexpected problems will probably occur.

2 If you occasionally need to transport large and odd sized loads, and cannot rely on the availability of a truck, this design will do the job!

3 Total costs. including paint: E 55.- (for the trailer, not just the upgrade).

4 This trailer works great for me!!! I have at least once a month some odd sized load to transport.
For small loads, I have my transport bags and my front transport carrier.
I will still need an intermediate, 2 wheel trailer, for taking a welding transformer, a power generator or gas powered water pump, but it would also be a nice challenge to build a bakfiets (riksha for cargo).