I took the space saver wheel out of my 07 Honda Civic and tried it on the front of the MX5. It seemed to fit OK and did not foul the caliper but not confident it was sitting flush against the hub although it felt solid once the nuts were done up.
I took a couple of pics, let me know your opinions as to is it safe.
Martin
As far as my little wheel database can tell me the Honda centre bore should be 64mm and the mazda one 67mm. This means that your wheel won’t actually be bolted up hard against the brake disc face. If the centre spigot of your hub (the round sticky outy bit) isn’t sticking through the hole in the centre of your wheel then your wheels is not safe, not even a little safe - it’s just clamped up on the end of the tiny spigot and as the wheel rotates the bolts will slowly undo until it falls off.
If you are unsure get this checked out by someone who knows what they are doing. My internet sourced information may be wrong (I hope it is for your sake), so please check the fitting properly.
Also some cars have different bolt cone angles for steel and ally wheels - check your bolts are right for that wheel or again, the wheel could come loose.