This raft juniper weighs about 95 lbs. No sagging with horizontal supports screwed to cross supports. The length is about meter long. If I made a long bench I would do supports along the length and depth.
The bench is used mostly for display. The screen behind shades it until afternoon and the afternoon sun hits it for 6 hours. I am a big believer in watering the benches and back screens as well as the soil. The stone drip area and cement building blocks evaporate to make a micro climate. I have never had the surface feel hot to me. If the color were darker it would absorb more heat. I think the only negatives are the weight and the texture holding dirt. I use a regular hose jet to clean. I am not fond of power sprayers. Too forceful on wood, stone or siding.
Dimensions were 1.5 inch and 3.5 inch trim boards. The boards are prone to breaking and must be supported during transport and cutting. My table saw blade and circular saw blade worked fine. Eye protection and a mask are needed to protect from cement dust during cutting and shaping. Send photos of your benches when they are done please!