What if you put them at the same height? As Uncle Dave pointed out, you could do all your molding on the ground, but a molding bench would raise the work to a convenient height. You could raise the sandbox to the same height as the molding bench. But isn't the molding bench itself basically a box full of sand? Could you pour over it? I don't see why not, and once the pattern was drawn and the flask reassembled, you wouldn't have to move it at all.
But that would require lifting the crucible out of the furnace, raising it above the bench, above the flask, and then pouring. All that extra movement of a glowing flask filled with molten metal sounded like an unnecessary risk. So eliminate it. Expand the benchtop / sandbox to make room for the furnace. It needs a bed of sand 2" thick, and maybe some firebricks to rest on and to set the lid on. But won't it be awkward and dangerous to lift the crucible to shoulder height or above? Will you even be able to see the sprue as you pour? If the mountain won't come to Muhammad...
Build some steps near the furnace. Stand at ground level to prepare the mold, then climb the steps to melt and pour. Ideas were coming thick and fast now. I revised my plan through many drawings, and made further changes during construction.
Something I learned when I worked in the shipping department at Rust-Oleum: when stacking rectangles of identical size, the stack will be more stable if they don't line up exactly. Because the layers alternate directions, each block rests on two blocks. Because the layers don't quite line up, each block also rests partly on a third block.
The base is 48" x 48" x 24" with 8" steps.
I used salvaged wood (mostly laminated particle board) for the bottom of the sandbox.
Some good quality dimensional lumber turned up on the curb just in time for the sides. I used 2x6s for the molding area. Since the furnace area only needs 2" of sand, I used 2x4s there.
As usual, spring clamps and electrical junction boxes held things square for assembly.
I vaguely remembered reading that concrete could wick moisture up from the ground, and that wood structures on a concrete foundation needed a water break to keep the wood from rotting. I had several blue tarps from Harbor Frieght's free with any purchase coupons, so I used one under the box, one to line the box, and one to shed rain.
I made a lid to keep out cats and weather.
Weights, bungees, and tent stakes hold a tarp over the lid to shed water.