From what I can see, if it's half the length of recommended, you will end up with twice the current for obvious reasons, as mentioned above, one way would be to follow the instructions , but wire the two windows in series.
Assuming your concern relates to width rather than height, that is a shorter heating strip drawing too much current;
I think I'd approach it differently, at least investigating an alternative method; the kits seem to have 0v on one vertical rail and +12v on the other, so one connector each side of the window.
I'd look at running both wires to the same side and split one connector bar in half. Stick that to one side, and the full length one to the other.
Use half the heating elements at the top and the other half to the bottom of the split connector bar, and all commoned together at the other end on the full length bar.
That way, you effectively have two smaller windows wired in series on each side.
Bill
Edited By peak4 on 10/10/2020 21:56:56