I believe it is (or has the footprint of ) the Stiller TAC 408/1.600. If so then I'd try to reach out to Scott McRee, founder of McRee Precision - which was bought by and product line at least partially assimilated into the Accuracy Solutions FAST product line. Current "Stiller" FAST models at https://accuracysolutions.com/shop/...I4MC4xLjAuMTczNTMyODI4MC4wLjAuMA..#cbdea2411d
That action (the Stiller TAC 408/1.600) is used for ELR shooting and therefore many rifles have been inlet for it. McMillan and Manners both have built many for that action, so have the inlet. Beyond that, McMillan sells flat tops and they or others can inlet it for you.