As those who have been following along with our "minimum blimp" project to create the cheapest possible autonomous aerial robot (by adding an open source autopilot to a toy RC blimp), maintaining altitude in the face of air currents and temperature gradients has been a continuing challenge. The main problem with the toy RC blimps we start with is that they have a single tiny vertical motor and prop to control altitude, and unless you've trimmed the blimp exactly right and conditions don't change, that's not enough to reliably keep the blimp off the floor and away from the ceiling.
The best way to increase the vertical "authority" or control power is to get the two differential thrust props on each side to also do some vertical work, by tilting up or down along with their usual job of driving the blimp forward, back and right and left. Such tilting props are called "vectoring thrusters" and they're what the expensive blimps use. But on the cheap toy blimps that we start off with, the shaft that holds the two horizontal thrusters is glued and screwed into place.
No fear. Converting this shaft into one that can rotate is a simple matter of five pieces of Lego and a small RC servo. You can see it work in the video above, but here are some shots to show how to make it.
Before (typical toy blimp gondola, with RC equipment stripped out):
That's it! Needless to say in the autonomous version the onboard autopilot will drive that servo, not a RC transmitter. But you get the idea. If the two vectoring thruster do the trick of altitude hold, I may remove the vertical thuster entirely to save weight (and two I/O pins). If I need even more vertical control, I'll keep all three going.