The valve linkage is in the back, where it is not exposed to damage.
Routing the extra plumbing in front of the pre-existing tubing offers some protection for the pistons and also makes possible a very long 4th tuning slide. This slide can be pushed in to make 12&4 exactly in tune, can be pulled out to make 4 alone in tune, can be pulled some more to make 2&4 in tune, or can be pulled out still more to make 4 alone work instead of 2&4.
I also made an extension on the third valve button; this arrangement is comfortable even for small hands:
The second valve slide now has a pull-ring and a small bungee cord that pulls it back down:
As you can see, I did not put any effort into making the instrument look pretty. My goals were comfort, playability, and cost control; it was a success in those areas. I have used this tuba almost every day for more than ten years, giving lessons to students and whenever I need a small tuba but don't care how it looks.