You can get one of these cavity filters off eBay and tune it to 978Mhz (or pay to have it tuned when you order):
High Power Low Loss UHF 900 MHz ISM BandPass Cavity Filter Repeater NEW
https://www.ebay.com/itm/High-Power-Low-Loss-UHF-900-MHz-ISM-BandPass-Cavity-Filter-Repeater-NEW/362807228239
Here’s how mine is tuned:
