Not strictly a "custom" cube, but I've imported Card Kingdom's Starter Cube into Forge for booster draft and sealed. A few friends and I have played it IRL and it's been fun. Unpowered and containing few expensive cards, it serves a good introduction into the world of cube, but it still has plenty of rares and combos to keep it interesting.
To import it into Forge, I had to replace 9 of the
Conspiracy cards that were draft specific and not implemented in Forge (stuff like
Leovold's Operative). I tried to replace them with similarly mana costed cards that attempted to achieve similar advantage when drafted; some replacements are more successful than others. However, I feel the pool remains well balanced and pretty fun.
This is version 2 for those who are keeping track. Drag and drop the \res\ folder into your Forge \res\ folder. When you've done that select either Booster Draft or Sealed Deck and choose custom cube, then Card Kingdom Starter Cube v2. This is designed for 3 booster packs per player; Forge will probably error out if you try to do sealed with more than 3 packs. I think it plays best if you draft 3.
This is cubetutor # 103470.