Accumulators enable succinct membership proofs and are critical for credential revocation and other verifiable services. In the post-quantum setting, hash-based accumulators remain transparent but incur \(\log n\) overhead, while RSA alternatives achieve constant costs but are not quantum-safe. Lattice-based constructions from the PASS lineage provide constant-size proofs with transparent setup, yet existing candidates either require trapdoors or yield large witnesses. We present COMPASS, an improved lattice-based accumulator that is trapdoor-free, quantitatively analyzed, and fully implemented. Our proof-of-concept demonstrates constant-size witnesses as small as 4.3 KiB and verification times as short as 3.6 ms at PASS \(_G\) -style parameter points, maintaining the 128-bit BKZ-GSA security margin. Compared with our re-implementation of Maeno et al., COMPASS achieves up to 52% smaller witnesses under comparable nominal parameter scales. The design enforces strict norm bounds, weighted rejection sampling, and domain-separated challenges, and its accumulator configuration limits long-term key exposure, mitigating known PASS-lineage weaknesses. These results show that transparent, post-quantum accumulators with compact proofs are practically attainable. COMPASS thus provides a reproducible, bandwidth-efficient foundation for credential-revocation and IoT verification systems requiring constant-time membership validation.