all cakets have diffrent sizes. 2 of the ame model might even be off by 1/4" to 1".
You can make it now, and when the time comes, bring it in. Fabric color lots do change.
what are your plans for the head insert? they can be covered, or just add an insert.
on wood caskets, it's stapeled in. on metal, i just pops in. any questions, i wil try to answer for you.