If this will help:
Material: regular .1mm sheet metal, lengths as needed; small angle iron for flanges, 6 small (10mm) nuts and bolts, old tire tube for gaskets, silicone and a wire feed mig welder.
I cut the old firewall out (approx. 36” ) and built a box “firewall” to fit two extensions set into the rear trunk. The complete new firewall is approx. 7” deep (initial wall 3.5', box wall also 3.5”) into the rear trunk and the rubber insulator for the lid fits into the top of the metal "box".
Using cardboard to measure and fit makes this fabrication so much easier and better fitting to the contours of the rear area (good snug fit all round). Various holes for wiring cut in the cardboard first, then when happy, cut in metal. I found the setup for both pass and driver's side bolt in set up worked very well.
This set up gives me ~ 3 inches clearance from engine to metal. Also, good room for overflow bottle (but I changed that in another MR2v6 and set up the overflow bottle in the trunk for ease and a larger bottle).
Advantages? With this new firewall removed, replacing ANY parts (removing heads for gaskets, valve cover gaskets, plugs, etc.) SOOO much easier. Well worth the time invested.
And, as always, a shout out to Paul Woods for his initial work and sharing his knowledge with others attempting these V6 conversions. Thanks again.