First up, what competition are you doing????
If it is for CAMS circuit racing events, you don't need a window net as an MX5 is considered an open car.
http://www.camsmanual.com.au/pdf/10_gen_req/GQ10_Schedule_I_2013-2.pdfThe link takes you to the relevant part of the general conditions of automobiles for harnesses and nets.
Have a good read of the full general conditions for automobiles, there are 14 appendices and 2 sections detailing what you need to do to race in cams events, full door to door, for any category.
http://www.camsmanual.com.au/10_gen_req.aspIf you have a full cage with intrusion bars then the net can be mounted failry easily so the net iss attached to the forward stay loop and is attached at the bottom with the release mechanism, you throw the net up and onto the roof to get out.
Have a look at what other owners have done at CAMS events to mount nets if they are using them to get a better idea of what will and won't work.
The FIA roll bar padding will be flame resistant to the standards required by CAMS and the FIA, so it is best to run this. It won't be fire proof, but will take longer to ignite than most non-compliant padding.
Hope this helps.