Getting early plants, cuttings and seedlings of herbs

I am selling plants, mainly herbs. Last year I was way too late with my plants. I am in Australia so seasons are the oppopsite. What happens here is that August is the coldest motnth with night temperatures up to 5 under freezing (-5c) and the September it heats up very fast and everyone wants plants.
How can I start plants without a heated greenhouse to be ready in Spetember? I read about the muck bed but my garden is fill and I am not able to dig. Would and above ground structure do the trick? I have one dysfunct cuttings bed which is built with old roofing sheets and I could use this. But wouldn't it be too cold because of the metal? Would fresh horse manure do the trick or what would I use to fill it? would this structure be enough to get seedlings started? With cuttings I have the problem that my el cheapo tap timers cannot handle frost. Any ideas?

I found something here, not very professional but might be all I need: http://www.urbanfoodgarden.org/main/seed-planting-and-propagation/seedlings-heated/aquarium-heater-seedling-propagation-box.htm