I bought something similar to these from the same seller on Amazon that worked okay for my box sets. Not perfect, but the box sets fit inside of them. The ones I got were Polypropylene, but they seemed to be out of the XL. The ones available in XL are Polyethylene.
https://www.amazon.com/Sleeves-Record-A ... ems&sr=1-3They also have a sampler set of sleeves in different sizes without the flap though.
https://www.amazon.com/Sleevie-Wonder-A ... ems&sr=1-4