Unfortunately, the trailer I have used was inappropriate for the purpose. Overall, I got around 35kilos which was fine with the cart but the problem was with the 6 to 8 kilos of the three solar panels and my surfboard that needed to be balanced.
That balancing was needed because the top load stayed out of the dimensions of the trailer and if it is a bit on one side or another, my cart started wobbling. This was very easy to happen and all the weight (the load, the trailer and the bike) wanted to flip me and made me oscillating left to right while I was cycling. Thus, I have repackaged at Placa Espana (5km form the start) and I somehow managed to balance it. Then everything was fine for another 5km. However, at one point I was looking at my GPS for the directions while the bike was leaning on the wall of a bus stop. Suddenly, the cart flipped the bike and the result was bended PVC pipe of my roof and very unbalanced trailer again. The first was obvious and I managed to fix it, at least until the next PVC store where I can replace it. However, when I attached the trailer to the bike, it wanted to flip the bike again. I spend an hour to try to balance the cart but it seemed impossible to me. The final drop fall when I attached the trailer and I barely managed to keep the system not to fall. It was incredibly hard to pull these over 60 kilos so the bike would not fall again. I almost twisted my knee and injure Achilles tendon! Then, I decided that it was too much and I could not lose hours in this battle every day. Thus, I decided to give up and switch the trailer with a double wheeler that is stable from side to side and does not affect the handling of the bike when stopped.