C170b53,

Yes, Your memory is still correct

And, yes I modified the top cowl. In simple terms, I recycled the top cowl and flush rivited a new skin where the old cowl door seated to top cowl

. The original spot welded piece, on the inside of the top cowl was retained and used to provide backing and material for riviting the new skin in place.
Then, after measuring carefully, a hole was cut in the new skin near the upper left corner and a C172 oil inspection door flush mounted. It turned out better than I expected. It is still easy to check oil and remove oil cap to add oil through the smaller door. However, a negative is you can not remove, or check water in battery without removing top cowl.
Some positive safety features of the new smaller inspection door are:
1. New door is mounted aft of the engine air baffle - on low pressure side - and will not pop open in flight like the old larger doors are prone to do when the door edges, or door locks, become worn.
2. Top cowl is now stronger with less spots to rub and wear metal thin.

Guess next time I remove cowl for oil change I should take some pictures and post them.