Low pass filters on 4m remove signals from e.g. the broadcast radio band beginning at 88MHz.
QRP-Labs sell a Low Pass Filter kit for 4m that may be suitable for 71MHz duty with the Portsdown TX.qrp-labs.com lpf kit
Higher power duty filters. In CQ-TV 260 Charles G4GUO published a low pass filter using ATC capacitors to go in a die cast box.
Mini-kits sell a low pass filter kit described as a 75MHz 7th order Chebychev Low Pass filter.
Band pass filters may present a problem. The QRP-Labs 12m or 10m Band pass filter kits may be a valid point for starting on experimentation for your receiver application. The upconverter shown on this Wiki contains good filtering for receive already. Antennas-amplifiers off a bandpass filter covering 70-70.5MHz with published characteristic curves. PA3CSG has a home built band pass filter which will tune to cover 70.5 -71.5MHz.