Probalby just a Weaver T-01 base and some tip-off rings (about $25 worth of stuff).
My only additional comment would be, that becuase of the recoil in a 303 (as opposed to the 22LR that tip-off rings were designed for), I use B-Square's airgun rings becuase they have two screws on each ring where you clamp it to base (makes it easer to torque it down really tight) as well as a screw that runs down through the center of one of the rings into one of the holes in the Weaver T-01 base that you can use to insure the rings don't move during recoil. Really, any airgun ring would probably work pretty well, as most have a similar design to the B-Square's.
Best of luck,