As the old school card pool is quite small this wasn't possible if you did it in the standard highlander way of building a cube. It's pretty hard to build PowerMonolith in a cube with only one Power Artifact and one Basalt Monolith.
Building the cube
So I started by listing a bunch of archetypes I wanted to support and then added the important cards for those archetypes. The next step was then to change at least some of the cards to less played alternatives. Like changing a Counterspell to a Power Sink, and things like that.
Then I cut some of the archetypes that used too many narrow cards, one example is Atog, that needs a lot of cheap artifacts to work.
Last but not least, I tried to trim and correct the specific numbers of each card. I knew from the beginning that I wanted to have 4 of each dual to make it easy to splash and play multiple colors. I also wanted to have 4 of each widely played staple as their would probably be more than one player at the table who wanted to draft those. A couple of examples are cards like Lightning Bolt, Swords to Plowshares and Serendib Efreet. Then we had cards like Millstone, that isn't a staple but can be played in very many decks. It is a great wincon for control decks, it is a must in Field of Dreams combo and Sylvan Library players would probably also want one. Therefore I needed more of them than I first thought.
The plan after that was to have 3 of each card that was important for an archetype, and 2 of each good card that neither was super important or a staple. The cards that only got one copy each were the fun offs, cards that don't really see much constructed plan but are cool and fun old school cards that should be in the cube. A few examples are Dragon Whelp, Royal Assassin, and Desert Twister.
I also decided to only have one of all the restricted cards, except for Chaos Orb. The reason for that one in particular was that some decks need answers to things like Moat, and also because it probably is the most old school card there is. Also, as Guardian Beast is in the cube I wanted it to be possible for people to get that combo.
And talking about Moat, there are some evil cards like that one, that many decks almost can't beat. So I decided to only have one of each in the cube. For example, you will only find one The Abyss and one Ali from Cairo.