If you have low mana costs, lots of ramp, rocks and reducers, or don't need to cast your commander several times, you can lower the number of lands in your deck
Then, using only the contents of your Prerelease Pack, you are given about an hour to construct a 40-card deck
Building a Halloween Grist deck is easy enough: just pick lots and lots of Insects
Fetch Lands are especially crucial, because even though they're expensive, they can help get you the mana you need at a moment's notice
If you do end up getting a custom sleeve, though, you may want to double-sleeve them
The deck is really cool and interesting, and people seem to enjoy having it around