What kind of performance are you looking for? A 1/4 wave ground plane is quite small and inexpensive. Since it will be enclosed in an attic, a properly tuned UHF mobile gain antenna with ground plane kit would give you 5-6 dB of gain at a very affordable cost point.
Whatever antenna you choose, remember that you should feed it with quality coax. Your specific choices should be dictated by run length and TX/RX power levels. I'd probably start with a minimum of LMR 400.