To make one or two wooden lattice trivets you can buy flat lattice moulding at a lumberyard, cut the slats to length, sand the cut ends and glue them together. But to make a few dozen from free pallet wood for family, friends and co-workers you need to have access to some shop machines and to make some simple aids.
Shop machines needed:
Electric drill with 1" rotary hole saw
Pallet boards about 1-1/2" thick with all nails removed
Mineral oil (optional)
Scrap wood and plywood (optional)
Wire coat hangers (optional)
Slip joint pliers (optional)
Paint brush (optional)
Plastic bag and corrugated cardboard or newspapers (optional)
Step 1: Preparing the Slats
My trivets are 8" x 8" with the individual slats being 8" x 1-1/4" x 1/4" so any dimension references will be to my trivets. Adjust to the wood you have, as needed.
Plane the boards so both sides are smooth and the final thickness is 1-1/4". On the table saw, remove the rough edge from one side of the planed boards. Set the fence so the slats you cut are just over 1/4" thick. I can get eight long slat pieces per board. Each of the pieces ripped on the saw will later be cut into 8" slats.
Set the planer to about 1/4" and run all the long pieces from the table saw through to clean up one side. The smooth side will be to the outside (top and bottom) and the rougher, sawed sides will be the inside glue surfaces.
Back on the table saw, cut the long slats into 8" pieces. Save the short leftover pieces. You may come up with something to use those for.
Using a disk sander smooth the cut ends of the slats. On each end of the slat I gently touch each of the 4 edges to the rotating disk at a 45 degree angle. Using sandpaper, I give each of the four 8" edges a quick rub to remove any rough edges. The slats are ready to assemble.
Step 2: Making the Assembly Fixture Etc.
Before you start assembling you will need to make an assembly fixture, spacers, and, possibly, a finishing "tank".
For the assembly fixture cut a 10" x 10" piece of 1/2" plywood for the base. Rip some scrap wood to 1/2" x 1/2". These will be the outside walls of the fixture. You need two pieces 8-1/16" long and two pieces 9-1/16" long. Drill four 1" holes in the plywood as shown in the photo. These are to use if you need to push the glued trivet out of the fixture. Making sure you have the walls square and just large enough to accommodate the 8" slats, in both directions, glue the walls to the plywood to form a square.
Use 1/8" hardboard or wood ripped to less than 1/4" thick for the spacers. The spacers are the little sticks in the photo. The spacers only need to be 4-5" long. Rip them so they are just under 1/2" wide. You may need to try several widths until you find the right one. You will need eight of these.
A 9" x 9" baking pan will work for your finish tank. Otherwise, make a wooden one similar to the assembly fixture but with the inside of the tank at least 9" x 9", with walls maybe 1" high, and no holes in the plywood.
If you want to apply mineral oil to several trivets at a time, cut up some wire coat hangers and bend them to serve as trivet hangers.
Step 3: Assembly and Finishing
Place a layer of slats in the assembly fixture. Position a spacer between the slats to evenly space them. Make sure the smooth surface is down. Dry fit the top layer of slats and spacers with the smooth surface up. If everything fits, remove the top layer and proceed to glue each of the top layer slats in place, making sure to put a spacer between each slat. KEEP THE GLUE OFF OF THE SPACERS. If a slat doesn't fit quite right, sand it to fit or replace it with one that does. With all slats in place, put three slats on top, perpendicular to the top layer of trivet slats, and put some weight on them until the glue dries. I use a gallon of paint and two bricks. When the glue is dry remove the trivet and slightly round off the four corners.
When you have a few trivets done you can apply the mineral oil, or just leave the wood bare. To quickly apply finish to several trivets at a time put the wooden finish tank inside a plastic bag. The bag will hold the mineral oil and prevent leaks. A glass or metal baking pan doesn't need the plastic bag. Pour a few ounces of mineral oil into the tank and put a trivet into the oil. Turn the trivet over to get the other side into the oil. Use a paint brush to spread the oil onto all surfaces. Don't forget the outer edges. Hang the trivets on wire hangers to drip and dry for 24 hours. Put a couple layers of corrugated cardboard or lots of newspapers under the trivets to catch the drips. I made a sloping drain and put wax paper on it. The drips make their way back into the tank. See photo.
After a day the excess oil will have dripped off and the rest will have been absorbed into the wood. Remind anyone you give a trivet to that they aren't dishwasher proof since the glue might soften. A wipe with a damp rag should keep the trivet clean.