Abstract We introduce MF-Box, an extended version of MFEmulator, designed as a fast surrogate for power spectra, trained using N-body simulation suites from various box sizes and particle loads. To demonstrate MF-Box’s effectiveness, we design simulation suites that include low-fidelity suites (L1 and L2) at 256 Mpc h−1 and 100 Mpc h−1, each with 1283 particles, and a high-fidelity suite (HF) with 5123 particles at 256 Mpc h−1, representing a higher particle load compared to the low-fidelity suites. MF-Box acts as a probabilistic resolution correction function, learning most of the cosmological dependencies from L1 and L2 simulations and rectifying resolution differences with just 3 HF simulations using a Gaussian process. MF-Box successfully emulates power spectra from our HF testing set with a relative error of $< 3~{{\%}}$ up to k ≃ 7 hMpc−1 at z ∈ [0, 3], while maintaining a cost similar to our previous multi-fidelity approach, which was accurate only up to z = 1. The addition of an extra low-fidelity node in a smaller box significantly improves emulation accuracy for MF-Box at k > 2 hMpc−1, increasing it by a factor of 10. We conduct an error analysis of MF-Box based on computational budget, providing guidance for optimizing budget allocation per fidelity node. Our proposed MF-Box enables future surveys to efficiently combine simulation suites of varying quality, effectively expanding the range of emulation capabilities while ensuring cost efficiency.