I built mine with the raised wing tips and I think if I could do it over I would build them horizontal. It flies great either way, but looks better flat (IMHO).

The rudder I enlarged about 30% and that seems to be a worthwhile move. I mimiced the shape of the horizontal stabs and made a framed up rudder that was convex instead of the concave version on the plans.

I also opened a passage through the "dummy" cowl intake of about 1/4"h x 1/2"w for any spilled fuel to escape and to get air back to the rear carb on my OS-FS70II. Which is, by the way, plenty of engine for the Contender. Boooo yah!

Worst problem(s) I have had are canopy related. I had a battle tinting it and attaching it. Partially pulled off last flight and I have decided to remove the glue reside (J&Z R/C-56) and reglue with servo screws or #2 self-tapping screws to mechanically hold the thing down. I lack the canopy gene or something. I hate 'em. When the canopy started to flap I thought I was hearing aileron flutter and I brought her down so quickly it's a wonder I didn't tear out the undercarriage. Whew!