SOrry I haven't bee back on in a few days ot answer your questions. I was terribly sick
But all better now!
I covered the container like you would wrap just the bottom of a box (you know so it looks wrapped but you can take the top off without taking off the paper). I used industrial strength velcroe to secure the fabric to the inside of the container. Then I cut a small slit on the back corners of the fabric. This wear I slipped the seatbelt through lke you would on a child's carseat. When I buckled it in I made sure it was secured to wear the seatbelt makes the clicky noise.
When I bought the boys retractable leashed they came with a nylon choke kind of collar (don't wory I've never used it on them). I looped it through two small slits I made in the back of the carseat and sewed it shut. I didn't sew Buckeye's very well which is why his broke. Anyway, I then used a small clasps that I attached to the leash and then to their harnesses.
As far as the height, I bought a rubbermaid tub that was deep enough that I could put 2 pillows in it. The 2 pillows elevated them to about the height of what the store bought car seats claim. In my hubbys trailblazer it was plenty high, but in my Audi they are a little low still.
Well, that's that...if you have any other questions let me know. I had fun making them. Just be sure that if you try this you buy a heavy fabric for the cover that it will hold the short leash. I'm sure you could make a better cover by sewing and all that but I'm a terrible seamstress so I just sewed enough to hold them in and cheated on the rest.