Seems I'm not the only one - the miui forum also has someone who's done it.
I practised with an old sim and old sd but they wouldn't fit.
After asking "kunhandi" on HUKD (he'd done it for the honor 7) he recommend the sd to buy:
http://www.amazon.co.uk/gp/product/B0166RR86O
is what i used - 64gb version - it's very thin
To separate the metal of the sim from the plastic - i just held it very close to a hair dryer with tweezers so i dont get burnt,and the glue softens - the plastic just goes soft and can be peeled off - careful not to force to much - dont damage the sim!
Next i made a mistake - i sandpapered the sd to make it thinner - but it ruined the card - DON'T DO THIS! no need.
Next attempt - i gently sanded the sim - just to remove excess glue.
Now with a spot of super glue, carefully glue the metal of the sim to the sd, in the right spot - i held it tight for a few minutes to make it as flush as possible.
next i found the combo would fit in the phone fine but the phone didn't read the sd.. i cut a small piece of sticky tape (approx 1mm x 3mm) and stuck it on bottom of sd under where the contacts are as i think they were being pushed down too far by the sim - pushed this all in and it works!
Its snug but i can remove it easily.