It is true that if text is clear on simple piece of metal X-ray might pick it up. However it is very unlikely this would actually happen. Your suggestion to use layered or jumbled metal pieces would make text unreadable airport X-rays are made to find dangerous items not these kind of tiny words and with so much luggage no one is looking for or would likely even spot such small details. And it is far too difficult for thieves to get our seed this way compared to other methods so unless our seed backup looks like weapon we are probably safe.