Several of these will be able to be embedded into a plastic back cover and thus not add any bulk at all, assuming that you use a plastic back cover to begin with. The LEGO method is actually quite nice; you find some LEGO pieces that are the same color as your case and completely flat on one side. Embed these in the back of the cover with the end that can "accept" pieces exposed. Then all you need to do is mount corresponding LEGO pieces to just snap it in place. Something like 4x 2x2 pieces makes for a very sturdy mount, but a larger plate is easier to embed and make look nice.
----------
Managed to dig up some images of the LEGO method. I did some prototyping a while back for case mounting solutions, and the LEGO system was nice, but I ended up going with a slide-in holder. You can obviously make it look better than my rough prototype test:
Image
Image