The original miniatures hill rules:
In the miniatures rules in the old Master Rulebook (I don't have the latest battletech rulebook) it says slight inclines cost no extra mp, but if it's too steep for a miniature to stand properly, it costs +1 MP to go up that bit of the hill.
Adjusted 'stackable hex' hill rules:
Because it's stackable hexes, there's no way a figure can stand balanced on the edge of any of them, so I've altered the rule to: if it's just 1 stack higher, it's a gradual slope, and no movement penalty applies. If it's 2 stacks higher, it's a steep slope, and a +1 MP penalty applies. If it is 3+ stacks high, it is 'cliff' and cannot be walked up - jump jets would be required.
This works out well, because 2 stacks high is about waist high on a mech, and 4 stacks high is LOS blocking, so it is the equivalent of 2 stacks high as 1 height level, and 4 stacks high as 2 height levels, with single stacks being gradual sloping between height levels.
I like the stackable hexes, because you can make different hills every time, or even combine them together to make less but even bigger hills. It will look better when they are all flocked of course :)
And a touch of Red vs Blue!