Nice work! I'm really looking forward to seeing what else you end up making!

If you check around on Yahoo Japan Auctions, there is one design that ties into the structure up front near the headlights as well. If you're so inclined, That would probably further help with rigidity.

Also, if you're up for making a version 2, consider extending the upright supports towards the centre of the car, and adding a second bolt like the OEM DC2 Type R bar. This should really ramp up torsional resistance from that member, since the joint there no longer acts as a pivot. Given the mass of the car and the forces the bar would likely be seeing (the towers are already part of the firewall, so I'm guessing comparatively low), you could probably replace the steel bar with an extruded rectangular aluminium tube and free up some weight.