This is on your new Xtrainer 300, right? I'm not trying to be an unhelpful smartass, but my suggestion is to adjust your expectations and/or spend some seat time getting used to the layout.
I'm 6'2" tall with a size 12 boot and have enjoyed 250+ hours on mine with the stock levers and have had no problems like you describe. To downshift or apply the rear brake, simply slide your boot down the stator or clutch casing until you contact the controls, and then continue pressing down. They're pretty much impossible to miss this way. For upshifting, just rotate your ankle slightly away from the stator case, then dip the inside of your boot down and then up in a swiping motion, as though you were using the shifter tip to wipe something off the side of your big toe.
If anything, my problem is staying
off of those levers when moving around on the bike in tight terrain. The worst is when I'm coasting down long steep downhill sections in neutral and I inadvertently put the bike into gear unexpectedly. Things sometimes get kind of interesting when that happens.
Whenever I'm not on the levers, and not moving around on the bike to negotiate rocks and steep descents, I hug my feet next to the casings on either side in what I like to think of as "home position" for whatever foot lever operation I might need to do next. From there, finding the rear brake or downshifting is easy.
If you watch the pro hard enduro riders like Graham Jarvis etc. you'll see that they do similarly... only with more grace, ha ha!