I noticed that there is actually a lot of unused space inside the Original Prusa enclosure, and I can always use more storage space in my small printing room. This shelf mounts behind the printer and adds a substantial amount of storage. The shelf can hold up to 4 spools of filament with room to spare, or boxes of parts and other items.

The shelf is split up into 3 main pieces to fit on the MK4/MK3 print bed. To keep the shelf from flexing when heavy items are placed on it, I also added support braces underneath. Even loaded with 4kg of filament, it barely sags at all.

Bill of Materials

In addition to the printed parts, you will need the following hardware:

M3nS nut (36x) - These are the standard Prusa square nuts, make sure you get DIN 562 nuts that are 1.8mm thick.
M3x12 screw (20x)
M3x16 screw (14x)
M3x20 screw (2x)
M3 Washers (28x) - optional

Required Tools

2.5mm Ball-End Allen Key

It would be technically possible to assemble without the ball end but it will be significantly more difficult.

1.5mm Allen Key

This is only used to push nuts into position so any similar sized piece of metal will work.

Print Instructions

I recommend printing all parts with PETG at 0.2mm layer height, with 4 perimeters for strength. Although it would look fine all printed in one color, I prefer to print the main shelf pieces with a color change at 1.2mm so the design really pops.

Assembly Instructions

Step 1

Prepare the Shelf Center printed part and twelve M3nS nuts. We will be installing them in the indicated slots.

Press the M3nS into the vertical slots using a 1.5mm Allen key. Note that the slots are not all the same depth.

Step 2

Prepare all 3 printed shelf sections along with twelve M3x16 screws and washers. Place the parts face down on a flat surface. We will be installing them in the indicated holes.

Install the screws with washers to connect the sections, but do not fully tighten them yet. Once all of the screws are in place, press down on the assembly to keep it flat while tightening using a 2.5mm Ball-End Allen Key.

Step 3

Prepare the printed shelf and twelve M3nS nuts. We will be installing them in the indicated locations.

Push the nuts into the horizontal slots using a 1.5mm Allen key.

Step 4

Prepare the Support Braces and twelve M3x12 screws and washers. We will be attaching the braces using the indicated holes. Ensure the shelf is on a flat surface for this step.

Install both Support Braces and insert all of the screws and washers. Tighten the screws while pressing down on the assembly to keep it flat.

Step 5

Prepare the Back Brackets and four M3nS nuts.

Press the nuts into the slots using a 1.5mm Allen key.

Step 6

Prepare the Shelf, Back Brackets, two M3x16 screws, and two washers. We will be installing the Back Brackets in the indicated locations.

Install the Back Brackets as shown and tighten the screws. Functionally they can be installed in any orientation, but I prefer to hide the nut slots towards the back side of the shelf.

Step 7

Prepare both Handle Brackets and eight M3nS nuts.

Press the nuts into the slots using a 1.5mm Allen key (4 per handle).

Step 8

Remove the printer from the enclosure. While it is technically possible to install the shelf with the printer in place, trust me it will be much more difficult.
If you have handles already installed, remove them now. If you do not have handles, the cutouts for them will need to be prepared.
Remove spoolholder post(s).
Guide the PTFE tube out of the way by looping it over the door.

Step 9

Prepare the Handle Brackets and six M3x12 screws. If you previously had handles installed, you can re-use the same screws.

Install both Handle Brackets as shown. Do not over tighten the bottom screw as it can crack the acrylic panel.

Step 10

Prepare the shelf, two M3x12 screws, two M3x20 screws, and two M3 washers.

We are now ready to install the shelf into the enclosure. While holding the shelf in position, insert the two M3x20 screws through the washers and into the Handle Brackets. Next, insert the two M3x12 screws through the back corners of the enclosure and into the Back Brackets.
Once all four screws are pushed into the holes, you will no longer need to hold up the shelf. Tighten all 4 screws.