My 12lb dachshund loved my Prius, where the rear seat-backs fold wonderfully flat right up to the front center armrest, allowing him to partially come up to the console. Not so much in my M3 since the rear seats don't fold down sufficiently, especially with the front seats all the way back. I couldn't find anything pre-made, so this is what I came up with.
Put the passenger seat back far enough for your typical passenger(s), with some room for reclining.
Make a template using some thin cardboard.
Glued together 3 thick layers of cardboard from my new 65" TV box.
Once the glue dries, cut out the thick cardboard same shape as template.
Cover with some contact cement and faux leather (https://www.amazon.com/gp/product/B08W2B24VL/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&th=1)
The finished product fits in very snug against seatbacks, and passenger rear door when closed. Initially I had an inflated beach ball in the rear foot area to support it, but it wasn't needed. If your dog is heavier, you may need that, or some type of formed foam support or big pillow.
Make sure to use a dog harness, attached to seat belt receiver.
Put the passenger seat back far enough for your typical passenger(s), with some room for reclining.
Make a template using some thin cardboard.
Glued together 3 thick layers of cardboard from my new 65" TV box.
Once the glue dries, cut out the thick cardboard same shape as template.
Cover with some contact cement and faux leather (https://www.amazon.com/gp/product/B08W2B24VL/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&th=1)
The finished product fits in very snug against seatbacks, and passenger rear door when closed. Initially I had an inflated beach ball in the rear foot area to support it, but it wasn't needed. If your dog is heavier, you may need that, or some type of formed foam support or big pillow.
Make sure to use a dog harness, attached to seat belt receiver.