I would for-go the saddle, and use 4" thick styro, cut to about 20-24" in length. Hollow out the underside to form to the lid, and line with fabric.
Shave the top edges for roundness, then plan out where you want the plants. Next start in the center, working outward, and cut shallow, "saucers" for the pots to drop into. Add hyacinth stakes for those that have to lean and then a few spots of hot-glue (pot to styro).
Continue working out and around and finish with moss, or a few small oasis chuncks, that you can add greens into. Might be a good candidate to assemble on-site.
HTH