Found it after some more searching. You want to swap the create function (located in the team repository) so that this (modified to reflect your intended changes) is what you put in your service provider.
Spark::swap('TeamRepository@create', function ($user, array $data) {
return Spark::team()->forceCreate([
'owner_id' => $user->id,
'name' => $data['name'],
'trial_ends_at' => Carbon::now()->addDays(Spark::teamTrialDays()),
]);
// Return the team with the given ID...
});