If you have 'room' for it almost anything will work indoors. Not as well as it would outdoors, but it will certainly be usable.
The biggest difference, as you've found out, is polarization, horizontal/vertical. There is a big difference between the two, some thing like 20 dB, which typically shows up more with 'local' or short range stuff.
Full wave loops work just dandy when mounted horizontally! The are not a strictly "up/down" radiator. Their polarization can be changed easily depending on where it's fed and it's general shape. Square or rectangular loops can be switched between vert/horz by feeding them either in a corner, or the center of one side. There are a bunch of them used that way. Full wave loops have an input impedance typically something around 100 - 120 ohms, not 50 ohms. That means that feeding them with an electrical 1/4 wave of 75 ohm coax is a cheap-n-dirty way of matching them to your radio.
Try your dipole in an 'L' configuration. One leg horizontal and one vertical. Only 6 feet of vertical space in your loft? So bend the top couple of feet side ways. That makes the thing a 'one legged' vertical antenna.
You've got all kinds of options. Try the ones that strike your fancy.
- 'Doc