I'd make a cardboard template by tracing the tank, then use that to cut out two plywood pieces. One will be the top of the stand (3/4"), and one will be the bottom of the stand (3/4").
Then build a frame of supports from 2x4s, or 4x4s if you prefer between the two 3/4" pieces. The shape of that frame doesn't matter so much because the outside of the stand will be attached to the two pieces of plywood. I'd probably put uprights at the three corners, and one in the center-front. Connect those uprights with cross-pieces top and bottom. Put adjustable feet under the plywood below each upright. Attach thinner plywoood to the back sides of the frame for rigidity. Wrap the front with plywood or slats of wood. You can curve 1/4" plywood by cutting vertical saw kerfs half-way through the thickness of the plywood. Space them every 1/2" or so for a nice even bend. You can cut a door opening on each side of the front of the stand. (The curved doors will need to be reinforced with curved pieces of plywoood to hold their shape, and attach the hinges to an upright frame, not to the plywood skin on the front.) Put trim around the doors and along the top and bottom of the tank. Maybe a toe-kick under the front of the stand.
I'm sure there are many ways to do it, but that's how I'd start.