John,
I have a Ringmaster that I built 20 plus years ago and it is still solid and winning it's share of contests. Things that worked: run the motor mounts back past the leading edge, mine go 1" past. Use 1/16" plywood, run it back past the leading edge and 3/4" pass the end of the motor mounts (assuming you run the mounts past the wing leading edge). Use long setting epoxy to install the fuse/wing joint. Use 1/2oz/sqyd. fiberglass to reinforce the fuse/wing joint 1 1/2" pass the leading edge apply with long set epoxy.
Controls: Use a 4" bellcrank with cable leadout. Set the pushrod up on the inside hole with a medium size tail horn. Controls should deflect no more than 25 degrees ish. Use adjustable leadout guide, initial setup center of lines exit 3/8" behind the CG. CG should be set about 15% back from the leading edge at the fuse/wing joint. Use a 1/8" diameter carbon fiber push rod.
Tank: Get a snub nose uniflow (no pressure) 3 3/4 - 4 oz tank, mount it to have the back corner kicked out at least 1/4. Use adjustable slide tab mounts (I use two on top, one on the bottom of the tank with 1/4" slots, Brodak has these made up). Initial setting for the Fox 35 is to have the tank pickup tube lined up to the top first fin on the head (about 1/8" above the glow plug), the Fox likes this for even upright and inverted consistent runs. Most important is to run the uniflow vent line to a free air stream vent tab off the back engine bolt (see picture). This is the single thing that gets the Fox to hold a consistent setting, you take a 3/16" wide strip of shim brass or tin and make a half loop at the end, solder a piece of 3/32" tubing to it, set the height to be 1/4" above the fuse, drill a hole and install it under the head of the back engine mount bolt. Connect the uniflow vent from the tank with fuel tube.
Engine: Run simple fuel 10% nitro, 27% oil (50/50 castor/syn). I get 10% Brodak all castor 29% and 10% Brodak 22% syn and mix equal parts, has worked very well in my Fox). I like to use a Randy Smith FoxPA needle valve, it has the same reduced center diameter as the Fox NVA without the leaks, gives a reliable run. If you use the FoxNVA make sure to use a piece of fuel tubing between the indexing wheel and spraybar to seal the threads. With the FoxNVA check to make sure that the holes are both across the venturi and set so when you look straight down the bore of the venturi you can't see either of them.
Prop/muffler: As long as the uniflow vent is in the free air stream the Fox 35 will turn a 10x6 prop (it really likes the RMS prop) and run a muffler with no overheating problems (as long as you use good fuel, ie no RC stuff). Set the engine to run a rich 2 cycle and have fun. If you develop the burb on the outside corners let us know and we can send the stick in the bypass mod info.
Best, DennisT