I promised to post my bagmate that I got and finally have the time to take pictures of it. this is the medium size one. dimensions are 3.25 x 6 x 9". Here are pictures inside my 30 birkin and 32 kelly. this, of course, fits the 35 birkin and 35 kelly also.
I'd have to say this is a fantastic product and I rarely leave the house without it. Highly recommend it (and I do not have any affiliation with this product whatsoever). Great because it keeps my lipglosses, pen, keys, and cell phone readily available. I don't have to dig in my bag. But most importantly, it keeps the Bearn wallet well protected. I don't keep anything next to the Bearn wallet and since everything is in its separate compartment, the wallet stays clean.
Also, you can fit more, but this is what I normally carry with me. Anyways, hope these pics help.
Thank you for the pictures. I bought the Bagmate but have yet to put it to use. Like KellyBag I am enjoying the inside of my Birkin. After seeing how organized without taking away from the bag - I'll have to try it.