PUZZLEBOXX

If we write out the alphabet in capital letters, we see that the only letters that appear the same both ways up are -

H  I  N  O  S  X  Z

Now we need to think of words containing only those letters - which ALSO are spelled the same backwards and forwards (palindromes).

The only word I can think of, is -

NOON

We might decide to also use M and W - because they look like each other when flipped.  This gives us more letters - but introduces a more complex rule for using M and W .  You might do better but the longest I can think of is -

SWIMS

If you want something more complicated, then the following example (in lower case cursive writing) might give you something to think about.