a mix of plain potting soil and sand can be used, or vermiculite and soil (or sand) or all three, with enough water to ensure it is wet, clumps when squeezed but water doesn't leak out. You can place a heat lamp over the bin to warm the sand or place it in the actual cage if the cage has sufficient room.
Some females don't care if the sand is covered, some won't lay unless part or all of the laying bin is covered. A few inches of space between the lid and top level of sand is good, this way your dragon can move sand around without having problems with it bunching against the lid. Partially covering the sand is probably best. If you put the dragon in the container and close a lid completely, good to have several air holes in the lid and sides above the sand layer, to ensure it is ventilated enough.
Most breeders use 14-17" deep containers, to ensure there is room for 8-12" of sand/soil/etc and space for a lid or cover is needed. To heat the sand a low watt basking bulb. You don't want it so hot as to dry out the sand quickly, just enough to raise the sand surface to 70-80F is more than enough.
Some dragons also like a partial hole prepared for them, which they will just excavate further to lay eggs, others want to use their own holes and may dig several till they are happy with it.
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PHLdyPayne