This is a very simple adjustable width napkin holder. That is, it can expand or contract to hold more or less napkins as needed. It prints as is, without any supports, and it assembles easily as shown.
This design was inspired partially by Zydac's Extendable Napkin Holder. This model is a bit simpler and should also prove easier to print. Each half is 125mm (~5in) wide, 125mm (~5in) tall, and 60mm (~2.5in) long. Thus the holder can extend from around 50mm wide to almost twice that, ~95mm wide.
The model is designed without tolerances for a snug fit. As such, it can be a bad idea to slide the holder all the way together, as it may be difficult to reopen. If this happens, you may use a butter knife to slide between the cracks and loosen things up a bit until you can get a good enough grip to pull the pieces apart without breaking them.
Print each piece as is; there should be no need to reorient. Needs no supports. As mentioned above, there is zero tolerance between the parts, which gave me a very snug, but not overly tight, fit as the natural shrinking of the plastic lets the pieces fit together nicely. If your pieces do not fit together well, try sanding down the rails a bit until they slide together.