They didn't. I just opened my recently-purchased kit and had a look; two 3mm pieces of light ply laminated together. It "interlocks" into the fuselage sides, and has a couple of small pieces of tri-stock, but you can pretty much forget about beefing it up further from the back side; too much of a stretch and too much structure in the way.

The box says a .70 four stroke is acceptable. I find that a bit… optimistic, at the very least. No joy on the 1.20 Surpass I was promised, so I'm back to deciding what engine to get. I want plenty of power, but I don't want to bust that flimsy firewall loose with too much power.

This is annoying...

.