Cut a strip of mat board 7/8" wide and at least 9" long.
I know my sink is wide, but that's up to me. You can design your cabinets the width you need and the sink you need.
Cut 2 pieces a little less that 2-1/2". The opening is 2-1/2", we need room for gesso and paint. Don't cut more than a 1/16" less. The sides are 1-7/16". Glue the sides inside the back and front.
Cut a bottom to fit inside your sink.
Cut a hole for the drain. Mine's 1/4".
Set your sink into the cabinet base. If you need to trim the front edges of the cabinet on either side of the sink, try sanding the sink first to see if that will help. If it doesn't carefully trim the sides of the cabinet where the sink will go.
If you've made the sink too small you can "shim" out the front opening with narrow strips of card stock until the opening is right. This shimming is perfectly acceptable it's done all of the time is full-size.
The sink should be level with the top edge of the cabinet. If it's not you can add a shim to the shelf to level the sink.
Sand the top edges of the sink round, all the way round, taking off the two square edges. Sand the front corners round. I also sanded the bottom front edge a little.
Paint a layer of acrylic varnish on to seal. Let dry.
Now, to make this mat board box look like porcelain.
I've used gesso for years to shape wood into porcelain or porcelain covered cast iron.
I've used a piece of mat board to apply the first layer of gesso on the sink. If you have a spatula, use it.
Use a soft brush dipped in water to smooth out the surface as much at you can.
Let this first coat dry over night.
It's morning and my first coat is dry and it's got some cracking. That's all right, we will fill them in.
You can sand some of the rough spots off. I found using the soft wet brush while the gesso is wet is the best way to go, though. Not so much sanding after the gesso is dry.
I've applied another coat with a flat stiff brush to add more gesso and fill the cracks. I am going to use the wet brush to smooth it out.
I am adding gesso to the inside corners and the seam where the bottom meets the sides.
Set this aside to dry. You can add more gesso, smooth, let dry until you are satisfied with the surface.
While the sink is drying we will move onto the counter top.
I've got a 1/16" lip over the front, that's past the edge of the posts, so I am going to have a 1/16" past the sides. I'm going to cut the bottom layer of mat board 6-1/8" x 2". I want to make an ogee edge so I will cut the top layer 6" x 1-15/16". Only need the ogee on the front edge.
Sand round 2 short ends and one long side, that's your front. Glue the 2 pieces together keeping the un-sanded edges even and the small piece centered on the larger one.
*Note: I've been playing around with gluing mat board together and I think I've found something helpful.
When gluing two large pieces like the one we have here I used a DRY iron to set the glue. Be sure your pieces are stuck together and all excess glue has been wiped away. Set the iron to it's hottest setting and to dry, no steam, and press. Don't slide the iron, that can shift the pieces. I left the iron on for about 30 seconds, enough to get the heat through. I took the iron off and then I pressed with a block of wood, I used my tailor's clapper, (I used to sew A LOT), until the mat board was cool, maybe a minute or so. Seems to work well.
We need to cut out the hole for the sink. Center the counter top on the cabinet base and mark where the hole is on the front of the counter top.
If you can put your sink into the base, set the counter top next to it and slide the sink out to match the front of the counter top.
Now, measure from the back of the cabinet base to the sink.
In this picture I've got my cabinet base on it's back to measure. My back is slightly con-caved and I want my cabinet to fit flat against the wall.
Looks like the sink is 3/8" from the back edge of the cabinet.
Draw your lines to cut on.
Cut out the hole.
Let's do a dry fit!
To fill gaps around the counter top glue on narrow strips of card stock. Especially along the back where you can see the layers of mat board. Paint will not cover that up.
*Note: We've been layering mat board together to make ogee edges. There's no reason not to try a couple of layers of card stock for a layer and make a different profile. You can add different combinations of thicknesses to create new profiles.