I agree, adding vertical gussets would make the exposed part of the hinge even stiffer, you can always improve!!. In my Stingray, I ended up with some pretty healthy fillets so I figured what the heck.

I have attached a couple of pics, see in the second one, I quit trying to make the clearance holes for the hinges on the movable piece, and just made the radius part in sections, so those slots in the elevator will clear the gussets.....Miguel did a great job of explaining the problem in post 58.

I agree with Tom, I would rather just cut this stuff out and do it myself...I spent more time trying to get the hinges to work on the Stinggray rudders, I think it has the same issue as the Viper, the taper was not right ....to me this will be hard to do on any ARF and is a little much to expect at the price, IMHO....you want those killer fits you almost have to do it yourself.

Having said that the only "ARF" I have seen that had predrilled, perfectly fitting hidden hinges is a Tom Cook Firebird, never seen anything close to the prefab and fit/finish on any arf.