Hi Teddy! It is polite enough, however if you wanted to make it slightly more polite, you can alter and use: "please keep out of the way, I am carrying (...)". It will make it sound less personal.

We like to believe we are exceptionally polite in England so I would find this slightly too forward! I would say: "Could you mind your backs please, this is very heavy" Technically it would be a question (so it seems more polite) but it would really be said as a statement without waiting for an answer.