Battery shelf can be any that is compatible with the Leviton pins, such as Elk or others that are mult-format compatible. If you need to meet UL or be a listed assembly (which would be the more correct thing to do) or be professional, the part numbers to put the M1 in a non-Elk enclosure are listed on P 67 of the install manual and can be cross referenced. The plate has been replaced by Legrand 364715-01.
Drilling, tapping or removing the panel from it's OEM enclosure or mounting not using the components described in the install documents will void the UL listing, which insurers do notice in the event of a loss. A voided listing or non-approved installation methods are generally frowned upon by them and building officials if you have the install inspected, no matter what the intent or how neat the modification was.