The acrylic case for the HiFiBerry Amp+ is sourced from the German based company Watterott. If you just look at the parts, assembly might be a bit complicated, but with this guide it should be relatively easy to assemble it.
First take all parts out of the bag. You should have the following parts:
Now remove the protection film from the acrylic parts. There is a protection film on both sides! Remove it from both sides. If you don't remove the protection film, the parts won't fit and there is a risk that you will damage parts during assembly.
Mount the HiFiBerry Amp with the supplied Nylon spacers on top of the Raspberry Pi. Fix it with the metal spacers that are supplied with the case.
Add a single screw and a metal spacer on the corner of the base plate (see picture below).
Add the side plates with the USB ports and the speaker connectors to the Raspberry Pi/Amp+ combination.
Now mount these parts onto the base plate. This should fit with minimal pressure. If it doesn't fit easily, check the position of the parts and do not apply much pressure.
Add the other 3 screws and spacers from the bottom.
Add the other 2 side plates. This should also be possible with minimal pressure:
Add the top plate an fix it with the supplied screws: