I’d made a rather thin box for drain extension purposes last year, whilst waiting for other things to glue, dry, etc. In the meantime  I discovered the the stock I’d used (18mm) was probably a bit too thin and had started to warp, even in final glued box shape.

So to work with thicker material. 35mm treated pine should do the trick. And with an extra grate rim around the top, very sturdy! I’ll take it home and put a tin of bitumous paint on it to give it an extra seal.

Now all we need is some rain to test it out.