Making a drum in metric today would be unthinkable...

However, until the 50's and 60's, many musical instruments manufacturers in France and elsewhere used the metric standard.
At ASBA, it was maintained until the early 60's. It is in fact the arrival of the patent of the manufacturer of synthetic drumheads REMO (and EVANS thereafter), in inches, which changed the deal as regards dimensions, by imposing the inch as a world unit.
We do not produce metric drumheads because there is not enough demand for them compared to the investment in machinery that this would represent. Ironically, it is the REMO drumhead brand that has developed a metric range! You may find your happiness by going directly to its website.