how to extend Builder class in laravel 5.2

I tried to extend Illuminate\Database\Query\Builder to override various functions, but i cant make it.

I created a custom builder (CustomBuilder.php)

<?php
use Closure;
use Illuminate\Support\Collection;
use Illuminate\Database\ConnectionInterface;
use Illuminate\Database\Query\Grammars\Grammar;
use Illuminate\Database\Query\Processors\Processor;

class CustomBuilder extends Illuminate\Database\Query\Builder {

    public function get($columns = ['*'])
    {
        $builder = $this->applyScopes();

        $models = $builder->getModels($columns);

        if (count($models) > 0) {
            $models = $builder->eagerLoadRelations($models);
        }

        return $builder->getModel()->newCollection($models);
    }
}

And a custom model (CustomModel.php)

<?php
use DateTime;
use ArrayAccess;
use Carbon\Carbon;
use LogicException;
use Illuminate\Events\Dispatcher;
use Illuminate\Database\Eloquent\Relations\Pivot;
use Illuminate\Database\Eloquent\Relations\HasOne;
use Illuminate\Database\Eloquent\Relations\HasMany;
use Illuminate\Database\Eloquent\Relations\MorphTo;
use Illuminate\Support\Contracts\JsonableInterface;
use Illuminate\Support\Contracts\ArrayableInterface;
use Illuminate\Database\Eloquent\Relations\Relation;
use Illuminate\Database\Eloquent\Relations\MorphOne;
use Illuminate\Database\Eloquent\Relations\MorphMany;
use Illuminate\Database\Eloquent\Relations\BelongsTo;
// use Illuminate\Database\Query\Builder as QueryBuilder;
use Illuminate\Database\Eloquent\Relations\MorphToMany;
use Illuminate\Database\Eloquent\Relations\BelongsToMany;
use Illuminate\Database\Eloquent\Relations\HasManyThrough;
use Illuminate\Database\ConnectionResolverInterface as Resolver;
use CustomBuilder as QueryBuilder; // MyModel should now use your MyQueryBuilder instead of the default which I commented out above

abstract class CustomModel extends Illuminate\Database\Eloquent\Model
{

}
?>

What is the correct folder to put these files, and how to call it from my models?

I tried in app/Extensions, in vendor, but i get the same error. Class CustomModel cannot be found.

Thanks

via Javier