Thee are lots of great jigs online but it doesn’t have to be anything fancy. There are 2 things that need to be accurate; height and twist. Using inexpensive dimensional lumber it’s easy to make sure these aspects are taken care of.
Step 1: Material
Start by getting a piece of 16mm MDF. You can go thinner, but the thinner you go the greater the chance of it moving or warping. MDF is preferable to Melamine because MDF has more grip naturally so you may not need to secure to piece.
In my case I used a 900mm x 600mm x 16mm piece of MDF.
The walls of the bay are made with 2x4s.These can be bought in lengths from any hardware store, but try to get the straightest possible. The height will depend on the height of the work piece. Keep the height of the wall just short of the total height of the work piece so you can add strips of thinner lumber which can be removed as you drop the router bit.
For the sled I used a piece of melamine so it would slide easily and I created a fence for the router out of 90mm x 19mm lengths. This will make sense later.
Step 2: Create the Bay
Measure the length of the work piece and cut the 2x4s to that length, plus 100mm in each end. Screw the 2x4 into the MDF making sure to pre drill so the MDF doesn’t bulge. I secured the second course using a block.
Step 3: Create the Sled
Cut a piece of melamine to the outside width of the 2 walls, and then add 90mm on each end. This extra length is to attach a 2x4 which keeps the sled on the walls. Then attach the rails with a gap between just enough for the router base to fit. I had a few millimetres play between the base and the rails.
Add some thinner strips where the sled will run along the walls. These will be necessaryto drop the sled by a small amount when the router bottoms out.
To cut the slot for the router bit use a circular saw and drop it into the melamine. This doesn’t have to be pretty, it just needs to allow the bit to come through and leave enough of the sled so the router base is stable.
Step 4: Using the Jig
Don’t take too much off with each pass. The smaller the cut the less tear out, especially when chopping instead of slicing. It's also to go as slow as possible to start to get a feel of how the bit is cutting. As soon as the router bit is extended as far as is is safe remove a part of the wall and lower it.
Step 5: Considerations
This takes a while so be patient. Once the routing is done you’ll still need to do a lot of sanding, so give yourself enough time. If you’re sanding end grain it will take even longer.