A client of mine is sitting with 20-30 used solid rubber forklift tires. He asked if I knew of a use for them and thought I would ask whether anyone here wanted to build some obstacles.
I have built normal car type tire obtacles before for my enduro bike, but you go through them, more than over because of the soft walls. Theses ones should be quite well anchored by their weight and even better if filled with sand.
They are free, just need to be collected in Kilarney Gardens.