Hi there!
I have a similar style booster seat from Amazon. Here's the link:
Amazon.com: Kurgo Skybox Blue Collapsible Pet...
I purchased this seat from Amazon and I'm happy with how stable it is once it's installed.
Also, taking it out and putting it back in is pretty easy. Just have to make sure you tighten the 2 belts around your car seat really well. The other really important point is that Mick didn't take long at all to feel secure in the seat. It's plenty roomie for him and his blanket...he weighs 5lb.
This manufacturer, Kurgo, has other styles and colors available; some which are less expensive.
You can't beat the price of the one you're looking at. Just remember to check the return policy of the vendor if you purchase the Solvit seat, in case you don't like it.
Hope this helps! Let me know how you make out!
Allie & Mick oxox