Having done one long side the last time, I managed to completed the two shorter sides. I don't recommend anyone tries this. Dry laying heavy blocks straight onto sand/soil makes for lots of lifting and relaying blocks as there is little readjustment possible once the block is in place. Using a bed of cement mortar gives you much more flexibility!
Before I left I did a check with the spirit level and was pleased - and surprised - to find the two incomplete ends of the base were level with each other! Just hope the whole thing is square and the right size when I've finished!