Depending on how big you want to make it, you can build a small structure off the ground. You just need to give it some supports. Once you've unlocked iron, you can make a stonecutter and place stone slabs which are pretty similar to building foundations in Satisfactory, minus the ability to make it a floating platform.

As a tip, you really gotta work that Hoe. Use the hoe a lot to flatten the ground as much as possible. Tap that hoe several times until the land won't change anymore. Then get that hoe to raise the land where it needs raising, or let her rest while you use a pickaxe to lower areas that won't go lower. Then work that hoe some more to level out the freshly changed area.

Remember that the ground tries to level itself to where you are standing, so stay in one spot and work from there to get as much of the area to the height that you want, and then repeat the process as necessary until everything is as you want it.

Hoe level to the height where your character is. When you know this, the hoe work just fine.

To flatten ground completely, in most cases, you need the hoe AND the pickax. Going back and forth between those two tools I have flattened large areas perfectly.