"Could you not just book it on as sport equipment. In a big bag padded with clothes or even a placy dive gear box.
Means as well that you get an extra bags worth of kit. Which isn`t counted against your weight limit"
Last time I flew with sports equip on easy (March) it got an extra 12kg on the allowance, so it does count (though i wouls guess that a m/cycle helmet wouldn't weigh all that.
Helmets are not accepted in hand luggage at some French airports (god knows why).