here is how you can mount the rudder tray
It`s tricky But the best way is to cut 1/8 inch off of each side and test fit ,,sand a little ECT.

from the Manual,

I recommend, for best results, using Futaba™ BLS Brushless servos,
or an equivalent. For elevator halves, use the BLS 651,or the BLS 351`s for ailerons
use the BLS 551. On rudder, use a 200-inch ounce servo for the most consistent results.
For clevises on the glow models, I recommend the Tetra™, Central
Hobbies™ or MK ™ BB clevises where possible, for all surfaces and
Central Hobbies™ Carbon/Titanium push rod systems. It is important
to have as little linkage slack or looseness as possible, for the
best trimming, flying and durable results. Remember, quality servos
produce quality results!

RUDDER TRAY<
#1 Test fit the tray, in the rear edge of the canopy opening. It
will need to be cut down slightly to fit the fuse properly (warning
do not force fit; it can ruin the outer finish and break the fragile
sides) Open the slotted area in the former slightly and trim the
tray, to allow for some adjusting. There may be some modification
required.
#2 When the tray has a good fit, install it parallel to the angle
that the cables will run down the fuse, and exit in the rear. Then,
using a pencil, draw a light line on the fuse to mark where the tray
will rest.
#3 Next, remove the tray and add balsa strips 3mm below the line marked, and glue them in with CA or quick hardening epoxy.
#4 Reinstall the tray and glue it in on the rails, using CA or half
hour epoxy.