If you don't mind spending $$ for the best... www.nunatakusa.com
has a bag that will not make you feel like a mummy - but it's probably the best bag made and you pay for that quality.
Look at the Arc Alpinist or Arc Edge. This is a very well designed "top bag", although the feet are boxed. The bag is essentially a quilt, with a mummy foot box to keep your feet from sliding out the sides. It has connecting straps on the underside, if the temp is warm enough, you unclip the straps and sleep under a quilt. As the temps get colder, you cinch the straps to pull the bag in around your sides. There are a handful of commercially available top-bags, but none approach this one in the quality of the design or the quality of the down.
My husband and I have been using two person top bags for 20 years - we used to have to make our own by cutting the zipper off a rectangular bag and sewing a sack at the feet. But two years ago nunatak made us a custom overstuffed double top bag and it's unbelievable. 32 oz for two people, good down to ~30 degrees if we're naked, lower if we wear a layer of clothes beneath. If you cycle with a sleeping-partner, a double top bag is certainly the lightest nicest solution you'll find.
If you're not sure if you like the top-bag concept, check with the company (that's Tom, it's a small company) about his return policy. I'm pretty sure he'll let you return whatever you order if you think it's not right. Be forewarned that if you order a custom bag, Tom is notoriously unable to deliver on the schedule he promises. You won't be disappointed with the product, just don't expect it to come when you thought it would, months late is not unheard of. If you order a standard product, or better yet from the clearance-sale page, then this should not be an issue. I bought a clearance-page Arc-Alpinist and it arrived in two days.
There's a great site for reviews of light-weight backpacking gear, many of the review categories are useful for bike touring as well since some of the equipment list is shared. www.backpackinglight.com
. That's where I learned about nunatak.
Good luck, Amy
PS - no I don't have any relatives at the company, I'm just so pleased with the single and double nunatak bag that I can't contain myself.