I think that replacing the pigtail aerail on the receiver with a DIY sleeve earial is the easiest solution.
Easilly made from a piece (~1.5m) of thin 50ohm coax (e.g. RG174U).
At one end of the cable, strip/remove ~30cm of the outer plastic.
Gently push the ground braid backwards over the cable.
For 315Mhz, trim the braid to 22.5cm and cut the core (with insulation intact) also to 22.5cm.
Push ~50cm of heatshrink over the whole lot, to keep thing where they are.
Strip the other end of the cable ~2cm.
Solder core to the point of the removed pigtail, and braid to a ground nearby.
Hang the earial on a skyhook.
Leo..