It Europe it may be hard, but in the US, with WAAS, the GPS altitude readout should keep you within limits. WAAS accuracy should be less than 50' almost all the time. The GPS will need to be in the pilot's field of view, of course, not mounted in the console. Something like a Garmin 330 or 430 mounted on the glareshield should work. On the other side of the world, I have no ideas.