I had the very same problem recently, creating an extender-board for a Documation M200 punched card reader.
The relevant mil.std is 21097 which has all the dimensions for the connector you slot the PCB into, but surprisingly nothing even remotely like a PCB-footprint. In particular you will want to look at the "M21079" documents.
The only place I found guidance on the PCB footprint was in old versions of EDAC's 307/357 series datasheet.
A really good "app" to supplement KiCad would be a edge-connector footprint generator, either as stand alone (python?) code, or as a webapp.