Have you tried using 2 bamboo nappies underneath, and forgetting about trying to boost a single nappy? I know this works great under wool so can't see why it shouldn't under fleece. But going back your the earlier posts, putting a disposable under wool, or PUL under wool or fleece won't work at all, as the wet won't be able to dry out of through the waterproof layer. Also, I've found that double layers of fleece, wool or a mixture make matters worse, not better - a single layer of breathable fabric outside works best. If you use a sleeping bag, make sure it's open a bit, as it needs to breathe as fast as the wool or fleece underneath.
I did a thread a while ago about how wool works. I'll see if I can find it and bump it, but in a nutshell, here it is
Wool is absorbent, but absorbs very slowly. Because of the body heat of your baby, and the warmth of the wool, it also evaporates, and this happens at the same rate as it absorbs.
When evaporation takes place, the wet is turned into gasses. Most people think this will besteam, but in fact it's not. Steam is created when evaporated water condenses again, and this happens when the air containing the evaporated gasses, hits something cold, and there is enough wet in it to saturate the air. (This is called the dew point)
If you have a PUL wrap, the inside of the PUL will not let the wet get any further, so the amount of evaporated gasses builds up, and also because PUL has a cold feel, the saturated air will cause wet to condense again, and get absorbed back into the nappy. This doesn't happen with wool, because it is warm, and can breath, so "dew point" is never reached, and the gasses continue one their journey, unrestricted by the wool , baby clothes, bedding etc. It will finally condense possiby when it hits the window of your baby's room in winter, but basically the amount of wet in the air never gets high enough to cause a dew point.