It looks nice. I assume the fully assembled photo is holding the post.

I'd probably plug weld the hole, but then you'll have some reaming to do, hopefully not too bad. I've never welded with aluminum behind my welding, but perhaps that would help, although filling a hole is different than simple welding.

Once you get done massacring the fame, you should send some photos to Bilenky.

You will need a pretty long seatpost if somebody ever chooses to raise it, but I suppose they can deal with that later.