What are those red things on the top? They look like wire twisters.

I just realized, on that website it does not say the spacing between
elements, only the length of the elements. How would I find out that?

The little red things on top are just plastic caps.

Element spacing & element size are critical.
Check the "Parts Catalog" & the "Instructions"
for element spacing. It's all there, even a photo of the bracket.

If you change the diameter of the elements, you will need to start
over with the spacing to tune the antenna.

As for Stainless Steel, don't bother, been there, done that.
Tried making some out of Stainless Steel CB Whips.
A very noticeable difference in performance.

You really want to build your own ?
Buy one from Arrow Antenna, copy it,
then send it back for a refund. Only cost you for shipping.
Got to warn you though, I have not got one back yet.

73 Al Lowe N0IMW
A HREF="http://ArrowAntennas.com/"ArrowAntennas.com/A
Arrow Antenna