While the two diodes in series would be adequate, I would still prefer to keep the diode across the positive to negative line, so it will let you know by blowing the fuse.
A bit redundant, but still a good idea.
Hey,the diodes are cheap enough.
I have far more than I'll be using for some good while.
Oh yes, another idea is to use a full wave bridge, so you can connect either way;
I had a printer buffer many years ago, that could have the power connected either way.