SDXC card that fits MacBook Pro?

My 2015 MacBook Pro (13" Retina) has 256gb SSD, and I want to add portable storage (256-512 gb extra) with an SDXC card, for audio & video files.  Amazon has SDXC -- several kinds from SanDisk & Samsung (low price per GB) with an “adapter”, plus Transcend JetDrive (higher price per GB) for 13" and 15" -- but I’m confused about what will work well.

     Why?  One web-page says “Custom Low-Profile SD Cards For MacBooks:  These specially designed memory cards unique to specific MacBook Pro models are low-profile and specifically sized to fit nearly flush on a Mac laptop's SDXC card slot without sticking out very much so that you can just leave the card in place on the go,”  and for MB Pro their diagrams show different sizes for 13” (it’s 16mm x 26mm) and 15” (18mm x 26mm).  So... can an adapter from SanDisk or Samsung (if it’s generic "one size fits all”) make their micro-sdxc card (11mm x 15mm) fit into either a 13” or 15” so it fits well but can be removed?