the portable object is (for example) the fire hydrant, an object that yuo wont see on the replay (replay system bug) use the dozer to move it in front of the roof target.
u cant go too far from it or it will disappear. use RAD/fugitive method to increase velocity with the MTB (with nrg i think will never work). there is also a system to increase alot the distance from the start of the runup and the firehydrant without making it disappear but its impossible to me to explain good luck finding it by yerself