I know you don't want to buy a new tent, but this one seems to be a pretty good design http://www.rei.com/product/802548. I own one but haven't used it yet. Fairly lightweight and it has built-in vestibule for your bike. Keeps the bike covered and near you at night. In addition to the bike vestibule, there's a good size vestibule on the other side. Not a lot of sleeping room in the main compartment, but that saves on bulk and weight.

I believe the current 20% coupon for REI members might apply, so that would put it at about $200.