Since I couldn't find anything like this, me and my dad decided to build one ourself.
This instructable does not include the complete instructions for the cabinet, but only for the part with the rounded doors since that is the most difficult part.
Step 1: Make a Mold
1. Nail 2 pieces of MDF together, so both molds will be exactly the same. Make sure the nails stick out a little so you can easily remove them later on.
2. Cut them to a rectangle that is much longer and a little wider than you need.
3. On one side of the rectangle, draw a half circle. Make sure the mold will still be much wider than you really need. Draw a center line on the molds. We will need this later on for alignment.
4. Cut out the half circle a little wider than you need. You should now end up with a mold that has an approximate circle that is a little too wide.
5. Shave the mold on all 4 sides to exactly the curvature you need. Do make sure that it keeps quite a bit longer than what you actually need.
6. Remove the nails to separate the two molds. You should now have to identical molds with exactly the right curvature.
7. Nail the two molds onto a third piece of wood, making an upside-down pi shape. The two molds should be spaced apart so that they are placed at 1/4 and 3/4 of the length of the piece of wood you want to bend.
Note: the molds can be re-used later on for top and bottom part (or interior shelf). The time it takes to make these two identical is thus not only for making a one-time only mold but also for making other parts of the cabinet.
Step 2: Kerf-cut Your MDF
Both pieces of MDF should be a bit wider and longer than you need. When calculating the length you need, keep in mind that the piece that will be placed on the outside will have a bending radius that is 2 * 8 = 16 mm larger than the piece on the inside. It is a good idea however, to keep both pieces at the same length, since it will then be easier to remove the fixations of the inner piece later on.
If you haven't done so yet, calculate the half circumference of the outer circle (= pi * (r + 1.6), where r is the inner cicle in cm). Round this up to whole cm. (In our case, the inner cicle was 19.0 cm, so the half circumference of the outer circle was 647.2 cm.)
Follow these steps to kerf-cut the MDF pieces:
1. Start by finding the center of the pieces. Draw a dashed line there and make sure it extends on the sides and on the back. We will need these lines to align the pieces later on.
2. Working from the middle, draw two rigid lines at 0.5 cm from the center line: one to the left and one to the right. Next, draw rigid lines at 1.0 cm interval to the left and the right until you at least covered the length of the half circumference of the outer circle. (It doesn't hurt to add one more extra just to be sure. In our case, I've drawn 32 lines to the left and 32 lines to the right, for a total length of 65 cm.)
3. Take a piece of spare 8 mm MDF and draw a few lines, 1.0 cm apart. We will use this to find the right depth of the cuts.
4. Adjust your portable circular saw such that the blade will make a cut of only approximately 7 mm deep. Test this out on the piece of spare MDF. If the cut is not deep enough, it will be difficult to bend and might even break. If it is too deep, the material will be too fragile. See images below for how shallow the cut should be and how far you can bend it.
5. Once the saw is adjusted to the right depth and you are satisfied with the test kurfs on the spare piece, make the kerfs on the real MDF pieces. Save most of the saw dust as you will need it later on to make the glue thicker.
After making the kerfs, fixate the MDF sheets with several glue clamps on some solid pieces of wood to prevent damage.
Step 3: Place the Kerfed MDF (part 1)
Next, remove all the nails, but leave the 4 screws. These 4 screws are all that fixate the MDF to the mold now. Check again that there is hardly any space between the mold and the MDF.
Step 4: Fill Up the Kerfs
1. Make a mixture of saw dust and wood glue (1:1 based on volume). If it is too thick to easily spread out, add a little bit of water. We used Bison wood glue D3.
2. Use a putty knife to work the putty into the kerfs. Work from the center along the cuts towards the outsides to prevent airtraps.
3. Once all kerfs are filled, dilute some glue (without saw dust) with a little water and use a brush to smooth out the grains of the glue / dust putty.
Place the second MDF plate on a flat surface and repeat steps 1 - 3 above.
Step 5: Place the Kerfed MDF (part 2)
Place the second plate on top of the first with the kerfs to the inside. Carefully align the plate on the other plate and fixate it with a lot of nails to the bottom plate and the mold. Try to minimize the gaps between the MDF plates and the mold, but let the nails stick out a little bit for easier removal.
The most likely part that will deform during curing of the glue is the space between the MDF plates where the straight section ends and the curve begins. To prevent the deformation, add some wood and glue clamps to hold the plates together.
Step 6: Cut Out the Doors
Then cut out the doors. During cutting, use the mold as support.
As you can see in the pictures, we used the large piece to create the base of the cabinet.
For the hinges of the doors, we cut out a small piece at the edges of the doors where the hinges should be. Then we inserted a thicker piece where we had cut out a hole for the hinge.
For the final step: connect the doors to the base of the cabinet and check that everything fits.
Step 7: Paint and Install the Cabinet
Once the paint is dry, fixate the cabinet to the wall, add the faucet and sink. Connect all the plumbing and then marvel at the result. ;-)