Don’t fret too much about it. I have a cube and it’s mostly a random collection of cards a friend and I had. Just separate the colours into roughly equal stacks and make sure each has a reasonable ratio of creatures to other spells plus a workable mana curve.

Play a few games and you’re bound to start finding combinations of cards you never thought of before. Then after a while you can tweak it if you find something is too unbalanced. For example, in an earlier version of my cube, enchantments were disproportionately busted, so we removed some and added some more removal.

One house rule we have is that if you pick a dual land (we just have the cheap ones), at the end of the draft you can exchange it for another from outside the cube that matches the colours you’re actually playing.