In No. 1 of 1982 of the journal in the article “Truck changes his profession” was told about one of the most simple designs of garden carts on Bicycle wheels, with enough capacity and good duct patency. If you already have a similar truck, without much difficulty, you can quickly turn it into a bike trailer. So, first take a section of pipe with a length of 80-100 mm, with a diameter of about 30 mm. at one end drill a hole that matches the thickness of the seat tube of your bike, and the other perpendicular to it make two holes for M8 bolts and saw out through the groove. There insert a rubber strip (as shown in figure 1), for example, old tires, size 50×150 mm, both sides of which must be the same opening as the coupling device and the drawbar of the cart. Now assemble the hitch and reinforce the seat tube.
From the side of the truck similarly flexible liner is inserted into a groove of the drawbar, which can serve as duralumin pipe with a diameter of about 20 mm; it should be curved so that the top end would be just at the level of the drawbar and the bottom flattened, wound up at the bottom of the box and fixed there: this connection must be detachable (so it was not difficult to implement a reverse transformation of the trailer into the truck).
If you are going to transport heavy loads, and even on bumpy roads or forest trails, take a double pole — two pipes. Bottom mount them on the bottom of the box and the rubber liner clamp between them as by bolts, but the configuration of the pipes have to change.
Now about devices for transportation of various cargoes. If you mean sand, sod, soil from the forest, logs, or, say, fertilizers in bags, here we can only recommend: do not overload excessively the truck to prevent damage, distribute the Luggage evenly PA square bottom or, conversely, concentrate in the center. But the branches of trees or bushes, no matter how fit, will be hanging from the low sides.