I built a 3 speed BMX a few years ago.

I had some steel tubing in my garden. was some kind of frame for a green house or something. I put another steel tube inside it. It was a tight fit inside the seat tube. Its stuck in there now. I stripped the bikes components. The frame went rusty. Crummy cheap paint I used.
I used a seat pin I bought in town on my Puch Minisprint. Its a regular BMX seat post. Cant remember the brand, probably DB as they have a lot of DB parts in that shop. The Puch is a little bigger than most BMX frames though.
Im 5 foot 8, or 1.7metre tall. The bikes fit me ok