Would it be permissible, in an emergency, to make one’s own garment part by taking a new white underwear item and sewing-in an appropriate mark or markings?
I’m aware of only one article of temple clothing that can be made at home and that is the apron. From Handbook 2, 21.1.42:
“Members may make their own temple aprons only if they use the approved apron embroidery and sewing kit that is available through Church Distribution Services. Other temple ceremonial clothing may not be made. Nor may temple garments be made.”
Obviously portions of the ceremonial clothing are not solely distributed through Church Distribution Services, such as white shirts, dresses, ties, socks, and belts (you can speak with a member of the temple presidency for any guidelines one these articles). Homemade garments are expressly prohibited. If you think your circumstance unique enough, you may ask your bishop what options are available to you.
“Information about ordering temple clothing or ordering garments for those in special circumstances (such as members serving in the military, members who are bedfast, or members with disabilities) is provided in Handbook 1, 3.4.”