That's it Gruff, just some 18 mm. MDF and a tight fit. I've had 165 kilos dancing on top of it (me and a mate; after a few beers, it seemed like a good way to test it), and it went nowhere.
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Alan Fluxion: I've done a dry start in several tanks now, from expensive enriched substrate with bacterial additives (as in this one), to soil-based tanks, and what I've found is that it has nothing to do with the soil (as long as it's rich in nutrients), it's more a case of enough lighting (mine is running at 14 hours with two reflectors), combined with a high enough temperature and humidity. Especially the latter is very important; in Spring, when the temperature outside was quite low, below 20 C at least, I used a cutup soda bottle filled with water to hold my thermostat heater, set at maximum, to release some extra heat plus moisture; worked a treat.
Another thing I've found, is that it's very important to remove as much standing water as you can; the substrate needs to be moist, but there must be no water puddles sitting on the substrate, as this is as good as flooding the plants, especially carpet plants. As you can see, on the lower right hand front corner, I've actually removed some of the gravel, so I can use a syringe to remove as much water as possible, so no puddles can form on the gravel.
Hope that helps! And as far as soil, I don't think I will ever buy expensive substrate again, and will be using potting soil from now on (Pokon Zaai- & Stekgrond works brilliantly, as you can see in the Propagator thread; it's comparable to John Innes No. 1 I believe, meant or plantlets and seedlings, extra enriched, all natural).; cheap as chips, and the plants definitely seem to prefer it!