A Japanese carrier-conversion reserve after Midway

After Midway, what would be the ideal cheap Japanese carrier alternative to the Unryu class?

Build a much larger reserve of rapidly convertible ships in underused yards during peacetime, potentially four or five dozen hulls.

The preferred alternative begins with peacetime shipbuilding capacity. Underused Japanese yards could have built many more ships designed for rapid conversion into aircraft carriers. A few ships already followed this approach, and the proposed expansion is deliberately much larger, roughly four or five dozen convertible hulls. This is a counterfactual preference for a larger conversion reserve, with other changes to Japanese planning left unspecified.