A Facebook quiz heralding the character you would play on “Downton Abbey’’ is fun, but what if an online quiz could be more useful? Answer a few questions about body type and sleeping preference, and Helix Sleep will custom-build your dream mattress using a blend of latex, micro coils, and foam. The company even makes split beds, in case one partner is a firm mattress-loving stomach sleeper, and the other a shivering waif who sleeps on her side and favors squishy. The mattress ships to your doorstep within 10 days, compressed in a cardboard box. You’ve got 100 days to try it, and unlike many other foam mattresses, it’s odor-free. Custom mattresses, starting at $600, online only at www.helixsleep.com