Hi
Yes, I am building with an MX5 donor. The tunnel needs a little modification to the profile as well as the dimensions, in other words it is fatter at the front but thinner at the rear. Also you might get some issues with clearance of the hydraulic clutch actuator if you don't factor this in early.
If nobody else has replied by the time I get in tonight I'll measure up and draw my changes - in the meantime check out my build site
http://www.mx5-locost.co.uk
Matt