Cedar frame for outdoor swing

I am planning on building a frame I saw made of cedar for a swing I recently bought (also cedar). The frame is square, not an A-frame.

I have a question about a specific joint on the fame, and how it is constructed. The joint is as follows: a 4X4 which composes the top horizontal member needs to connect to another 4X4, that it lies on top of, which composes the vertical member. But what is the best way to connect these?

It looks like the person who made the frame (from my memory) countersunk 4" screws about an inch, which would give about 1.5" of penetration in the vertical member. But is there a better way of doing this? The guy at Home Depot suggested 5" lag screws, but I didn't know if that was the best method.

Also, what type screws would be best? Are galvanized OK, or do I need stainless? The reason I ask is that the lag screws only come in Galvanized, which is why I was weary of buying them.


Thanks!

P.S. Also, what do you think I should coat this with? I was thinking varnish with a coat of Thompson's water seal. Is that necessary?

Lags are available in stainless also, just not at every HD, and NOT cheap. Of course if you've got doubts about a 5" lag cutting it, you could always supplement with joining plates (also known as hurricane ties in someplaces). They come in a variety of materials thicknesses designs, and coatings.

If you go back to HD, and look at their kids playground stuff, (or to their Simpson Strongtie section), you'll get an idea of what I'm talking about.

Thompsons should be fine by itself