In theory, there's an alternate solution to the many chips implementation mentioned previously, but this would require a much larger budget...I would buy a cloned 1mb L2 cache stick
Motorola did produce an integrated 256 KiB L2 cache for the 60x bus, the MPC2605. You can use 1, 2 or 4 of them to produce a 256 KiB, 512 KiB, or 1 MiB cache, and those would require minimal external support. And the chips are available from RocElec. However, those clicking on the link will have noticed the "this is for professional" pricing, at over $50 per unit...