Excellent work Marv you should be proud of them.
Glad to see you pinned the boards and did not glue them. The main reason for not gluing the boards is to allow for movement in the timber. It being soft wood I suspect you will see a lot of seasonal movement in the wood.
Had you glued the boards they could split at the weakest point or bow away from the frame entirely, this is also why you allow a gap between boards.
Your bracing is fine as you need to stay above 45
° to benefit from having them however for maximum strength you should notch them into the rails to prevent them from skewing.
This video explains it at 2.00
https://www.youtube.com/watch?v=D9g4c8-CYTY
Another tip is to try and alternate the grain direction on your boards, not always possible but can help to reduce cupping.
The bin bag shouldn't be a worry, leave it be if it can't be seen or pick it out with a chisel if you can.
Lastly make sure you put plenty of preservative on the end grain bottom of the boards as they will suck up water like a sponge.
Great work, well done.