40-145 will work

Hello there. I am not sure what you mean when you say "square stock". If you could clarify that for us that would be great. In the meantime, take a look at the diagram below of our 40-145 storm door latch to see if this will offer some additional help to you.

Thanks for waiting. Please see our 40-187L. This is a 4-1/4" long by 3/16" thick spindle which fits our 40-145.

You will need to use a hack saw to cut it the correct length. Watch the related video on installing the 40-058 handle. It's a different set, but the principle is the same. In the video they suggest having the spindle extend past the storm door approximately 1/4". I would cut it a little longer to make sure you don't cut too short. You can always trim a little more off if it's too long.