I would use the solid spacers over the extender style any day. With the extender there is nothing but the bolt centering the wheel. You can't tell me those things don't flex around hanging out there in the wind.
Where are you located? See if you could find a machine shop that would take and end mill and machine the pocket a little deeper so you can use stock studs. Then ones I made for my kid, I use stock studs in the hubs and the spacer both. He beats on it a little and it takes it all.
I would make them fit for you cheap, but the shipping back and forth might not be worth it.