To be quite honest, to cast these is going to be a lot of work. First, you will need to create a mold for the shuttles. The way they are shown in the pictures is how I would try to mold it. The high points in the center of each side will be the entry point for the resin.
As for a resin, I would go with a polyurethane. They are durable, but can be flexible when cast in thin spots. The Alumilite Amazing casting resin would be my choice here. https://shop.resinobsession.com/collections/resin/products/alumilite-amazing-casting-resin-16-oz-opaque-white
Perhaps an easier way for you to create unique tatting shuttles would be to buy ‘plain’ ones, then decorate the domed sides with your flowers and other items. Then use a doming resin to give it a glossy finish. The Resin Obsession crystal doming resin would work great for a project like this. You can buy it in our store here: https://shop.resinobsession.com/collections/resin/resin-obsession-crystal-doming-resin