I'd at least put something from strut mount to strut mount. That bulkhead that separates the trunk from the engine compartment/interior seems quite structural.
If it were me I'd take a piece of tube steel and make a mount at the left front of the engine compartment and run it to the RR strut mount. Then reciprocate it for the LR strut mount. Then make another tube steel mount from the RR strut down to the top of the LR subframe connection (and again for the LR side). Then a final one to connect the tops of both strut mounts.
You won't gain much weight by using some small tube steel but the rigidity will help more than you can imagine.
It does look cool without the trunk.
