Yes, that part is very difficult. Even foam wing shucks are not exact. I block up the wings by using two identical or as close to identical ( block sand them on both sides with a sheet of 150 grit sandpaper adhered with two-sided tape to my slate building table ) pieces. The foam blocks and several pieces of sheet balsa shims ( 1/32" and 1/16")handle the blocking chores. I recall that Sam Turner of Tampa, Fl. went into how to cut a fiberglass fuse for the wing tube socket and corresponding tube alignment in a video he produced back in the late nineties. Now a days, guys are setting up inexpensive lazers ( there have been posts here in the pattern forum) which give incredibly accurate results.

An accurate , large surface such as a piece of pool table slate -one that is long & wide enough to accomodate the fuse with the wings is the ideal. I have half a pool table slate. Problem is lifting it and you need a very stable table to support it. Pool table companies usually sell damaged pieces and may even cut one length wise in half for you.