Thankyou, that looks good, however, I've stumbles across a solution from found bits around the garage. Cut two strips from a plastic chopping board that together are the size and shape of the box section legs, marked a right angle a couple of inches in, then sanded to that line with a flap disc on an angle grinder, a bit of tweaking and I got the two bits back to back slotted into the leg with a nice firm fit that won't fall out by mistake. Then it was just a case of transferring the angle from one of the other feet, about 10 degrees as an estimate, and cutting one piece to that angle, the two bits were then put back in firmly and I could then run the saw across the second piece. This has worked perfectly, its solid, hard wearing, non marking, waterproof, and free.
