Posted: 10/01/2015 at 14:50
Homebase or a specialist timber merchant may be willing and able to make those cuts for you. If you have to, you can do it yourself with a carpenter's saw. It may be a bit scruffy, though.
If you do a sloping top with a long diagonal cut, you make right-angle cuts to cut your wood down to size and then put on, from the bottom up, in the case of the 100cmx120cmx525mm and 675mm example, 5 straight pieces 100cm long across the side then one piece that's 50cm straight and 50cm diagonal and one piece that's just 50cm diagonal. To secure that top one, you put a length of your 25cm-square wood along the diagonal. It's going between the corner posts, so it's only doing 850/1000 of the length of the diagonal, so its length is SQR(850^2 + (150x0.85)^2) = 859.5mm, and you need to cut the ends at a slight angle, 3.3mm off the perpendicular if it's 22mm wide, for the ends to fit snugly against 75mm posts.
Using 100mm boards, 500mm front and 700mm back, 5 boards across the front, 7 boards across the back, 5 straight boards and two tapered ones just the same at each side. Sloping rail 855.9mm sides length, 4.4mm offset, 860.3mm total length. Top-down approach: 1020mm length each side, 20mm offset across each end, total length 1040mm.
Why yes, the birds did come round to congratulate me on the neat job I did on their house.
Now for the alternative: putting the scruffy edge at the bottom where it won't show once you sink this thing into a sand bed. This works pretty much the same way but your side planks have to be 1011mm long at each side not 1000mm long, and cut at the ends 11.25mm off square, for total length 1022.43mm. Doing it this way, you start at the top and put on whole planks from top to bottom, then add you diagonally-cut planks and the rail behind them at the base.
Fastening them together: you can use nails, which is cheap and simple and works fine, or you can use screws, which gives you the option to dismantly and reassemble the thing a few times. If you're using screws, drill a hole just wider than the screw and its thread through one part, counter-sink the top of that hole if you're using counter-sunk screws, then assemble everything and drill holes the same size as the screw shaft not including the thread into the other piece of wood, and when you put them in tighten them one half-turn each at a time, rather than tightening one all the way before you start on the next.
So far, we've got four walls in something like a rectangle. If it's actually a rectangle, which you want, the diagonals are equal. The way to make sure this stays true is to brace it diagonally. If you did the "top-down" boards on the side, you can fit horizontal rails at the bottom front and back to match the ones at the sides, then put diagonal pieces across them at the corners. Whichever approach you took to the sides, you can also tie it rectangular by threading thin wire through hooks in the corner posts. Pull them tight, measure the diagonals, adjust the wires as necessary until the diagonals are equal and then tie them securely.
You also need a lid, a hinged lid. Put your piano hinge or two door hinges on the outside of the top rail on the back. You should probably do that before you assemble and position the walls, actually. In fact, you should have this part set up ready to screw back together before you assemble the walls, because it's a bit awkward trying to line up hinges and fight gravity and everything else. Let me think this one through again.
Okay. You make the lid, lay the lid upside-down on your work table, put the top rail along one edge of it, put the hinge(s) on the lid, stand them up against the top rail, screw them to both, check that it does fold back and forth, unscrew the hinges from the top rail and leave them attached to the