Maybe try an Airwrap? Keeps the limbs from getting between the bars.
Do you use sleeping bags? I find my DD wriggles and moves around her cot a lot if she is not in her sleeping bag.
Both of these options are safer than cot bumpers.

It could be just a stage, like you say practising at night - which could go away in a couple weeks. If not an Airwrap or sleeping bag might help.