I'd make a stone one - its really easy. I've just built myself 2.5m x 4m one and I'M A GIRL!! Mine was complicated because a) I wanted an irregular shape, b) its big, so needed an interior breezeblock retaining wall first, c) I wanted dry stone walling around it with stone capping and d) I had loads of old rough limestone I wanted to use, but this did not come in anywhere near uniform shapes and sizes. If you're after a rectangle and don't need double thickness walls, I'd get some stone and go at it. I dug a trench of about 5 inches deep and 4 inches wider than my total wall thickness, then filled with readymix concrete. Gave it a few days to go off. Then squished some mortar on top and started building. The breezeblocks were a doddle, being uniform, and the whole lot took me about half a day. Btw, I used readymix mortar from B&Q, but subsituted some of the water for liquid latex (from builder's merchant) just to give it a bit of extra frost/water-proofing. Its the stuff they add to render for the same reason. Makes the mortar stickier, so easier for a novice to use as well. Easy peasy. Putting a wider stone slab (eg. the foot square ones) on top as a coping works well, and it hides the edge of your liner if you overhang in by an inch or so.. Honestly, my boys' lego is more complicated. I honestly think anyone can be a brickie if they're only doing low walls - its staying true going upwards thats the tricky bit!