I just switched over the control parts of this mountain bike (Cube Limited 2011, a hard tail mountain bike with hydraulic Magura brakes) for left handed use.
Together with other works I did this over the course of the last few weeks, as it involved a bit of testing, finding the right position for the controls and trial and error. Also I used this to establish what I want for my road bike (currently work in progress, see future post). As I will have been getting quite a number of extra parts to set up the road bike (thumbies, cable stops, special brake levers, new brakes, etc), not using any particular extra parts for his one is a feature too.
One important setup tweak is to slightly tilt the saddle backwards.
For good stability particularly down any slight slopes or downhill, you want to slightly stand on the pedals, knees bent a bit, saddle firmly squeezed between thighs, to control your center of gravity to always be exactly over the bottom bracket [Tretlager].
Regarding steep downhill slopes, I am still figuring out the best way so not there yet with this one. In the olden days, I used a Blackburn rack to sit low down behind the saddle, but nowadays there are different options.
I put rubber grip covers on both handlebars. I got the variety that fixes to the handle bar with screw fastened metal rings rather than being too narrow,and rather by fixing them with glue. With too many items getting out on these handle bars, normal width grip covers would be too wide so I removed a middle
This (picture below) shows the right handle bar. The rubber grip cover was obviously modified (shortened). Adjacent to it is the Mert hand's handlebar adapter. There, correct position and angle was important for me as over the course of 1/2 hour or more, back pains may occur if the position is not good. Obviously, length of arm, length of prosthesis, flexibility of the hips and back all play into what works in terms of optimal bike posture. All I can say is that I usually make these evaluations and modifications a trial and error thing. Once the Mert hand adapter found its correct position, I mark it on the handlebar so whenever I remove the adapter to use it on another bike, it does not take too much time to get it back into position.
Next to the Mert hand adapter, I placed the lamp mount. Next to that, I mounted the (front) shocks control. All these, I do not need to access (with the left hand) too often during a ride. I still am undecided about putting a bell there, most likely I will put the penguin toy there (as that does not rattle when going over uneven terrain) (bells rattle and that really sucks).
The left side of the handle bar (picture below) now is full with the shifters and breaks. From outside in, first there is the grip cover. Again this is the variety that just slides on or off, and that is held in place with round metal calipers (I used red ones on the right and white ones on the left side). Secondly there is the rear brake, with the lever moved over from the right handlebar side where it originally resides. So I flipped it upside down. Immediately adjacent to it, but swiveled downwards, the front brake lever. I only use both brakes for faster riding, and I can still reach there with the finger tips. Then, I placed the rear shifter as I use that more often than the front chain shifter.
Regarding the next image, both front and rear shifter paddles can be reached but I rotated the shifters so that the rear shifters (placed more towards the left side) can be touched with greater ease as they are closer to the grip. Conversely, lifting the front chain shifter up on the bigger chainring requires a bit of extra pressure anyway, so making front chain shifting a more dedicated action was not a problem. Front chain lifting involves pressing down the big lever of the shifter towards the steering column (or, placed rightmost). I find it convenient to use the left hand to do an ignition key turn type movement. The urgent shifts usually occur when there happens to be a steep ascent behind a curve and these are all totally easy to manage with this setup. Particularly, descending the chain on a smaller chainring with the front shifter involves activation of the lever (rightmost shifter, image below) of a slightly smaller size than the lifter - and this can be triggered by kicking the lever either way, down or up.
I infrequently access the switch that controls the front shocks (leftmost switch, image below). The Mert hand adapter now has the right angle, but not sure I have adjusted all of the springs just yet.
A lot could be said about the very detailed angles of the two brakes and two gear switches - but really, you have to try out yourself. In some relative positions to each other, the gear switch levers may be blocked, say, by the body of a brake, or by some other structure. Some positions are easier to access than others. So, I try, change, try, adjust again, try, etc. until things seem OK. Then I take screw drivers and hex keys along at least for the first few rides. After a few rides, then, things are better.