We live in the desert, where it can be warm. We have French doors which we like to keep open at night to help cool our room and the rest of the house.

We also have dogs, and it is not safe to let them roam the neighborhood. Not to mention, of course, not wanting various critters to come in the house.

Strong screen doors are on the list, but for now, I needed a fairly simple and cheap solution. Another requirement is ease of installation and removable, especially in the case of fire or other reasons to easily get through the barrier.

Step 1: Plan

The french door frame is 2-in wide. This is a handy size, because a 2x4 is really 1.5-in thick. The basic needs are:

Must be easy to install the part with the slot for OSB
Must be strong enough to handle dogs or largish things banging into the barrier
Must be easy to make
Must have a slot for a 1/2-in OSB barrier
Must be easy to add and remove the barrier made of OSB
Must be high enough the dogs will not be tempted to jump

The plan, above, creates the part that screws into the French door frame. A screen we currently have across the doors is 2.5-ft high, which seemed to work well.

The tools are:

Drill press
wood drill bits
Screw drill
Electric drill (can double as screw drill)
Phillips screwdriver
3-in phillips screw bit
Table saw
tape measure

Materials:

(box) 3-in exterior screws (phillips)
(box) 1 1/4 in to 1 3/4 in screws (phillips)
Enough 2x4 good scrap for (2) 2.5-ft pieces
(1) 4x8 sheet OSB (per french door)

Step 2: Drill the Mounting Holes

You need two drill sizes: one for the 3-in screw threads, and for the head.

The first bit needs to be big enough for the head of the screw. See the plan. The order is:

Mark the 2x4's in 2.5-ft lengths. The kerf of the blade is not critical, so just mark at 2.5-ft lengths. This is a good time to trim the end of the 2x4 for a nice square-cut end.
Mount the bit in a drill press.
Make a line down one edge of the 2x4's, about 1/3 of the width (meaning about 1/2-in or 3/8-in from one edge. The exact distance from the edge is not critical.
Put a mark for the drill point on the long edge line 6-in from one end and every 6-in
Drill the holes 2-in deep. See the plan.
In the hand drill, put a long drill bit that is the same size as the threads of the screws. The intent is allow the screw threads to fit through the hole.
Using the previously-drilled holes, finish drilling the holes for the screw shaft.

Step 3: Cut the Notch

To cut the notch:

set the table saw to cut 2-n deep, 5/8-in from the side that does not have the drilled holes.

IMPORTANT: You want the notch on the same edge as the screw-head holes! See the plan.

Rip all of the 2x4's

Set the table saw to cut 1.5-in from the fence, to a depth 5/8-in. See the plan!

In both cases, use a scrap 2x4 to make sure the cuts are as expected.

Step 4: Cut the OSB

The first task is cut (rip) the sheet of OSB into 2-5-ft and 1.5-ft wide, and 8-ft long strips.

You should know the width of the door you want to mount the barrier.

Note: If the door width is less than 4-ft, you can cut the OSB sheet into a 4-ft x 2.5-ft and 4-ft x 5.5-ft parts.

How to figure out how wide to make the barrier itself:

barrier width = (total door frame width) - (2 * 1.5-in) - (1.0-in)

To make the barrier:

Cut the barrier OSB (the 2.5-ft wide strip) to that length.

To cut the handle (see the handle plan)

Find the center-line of the top edge
2-in from the top edge, draw a line at least 4-in long (2-in on ether side of the centerline)
4-in from the top edge, draw another line, 2-in on both sides of the centerline
draw a vertical line 2-in to the right, another to the left, of the centerline
The result should be a 4-in wide and 2-in high box.
Drill holes at the corner
Use a saber saw to cut out the box

Step 5: Assembling the Slots

With the scrap OSB, cut a 3.5-in x 2.5-ft strip per 2x4 notched 2x4

Take two 2x4's or a 4x4 at least 1.5-ft long. Make sure the 2x4's are lined up vertically and clamped to a work table. This gives you a simple alignment jig.

Put the 2x4 notched against the jig
Put the OSB on top of the 2x4. Align both long edges and the ends.
(optional: put glue between the OSB and the 2x4)
Put 1.25-in screws (or longer, but still shorter than 2-in) an inch or so from the ens to secure the OSB to the 2x4.

Put screws in every 3-in or 4-in trough the OSB into the 2x4

IMPORTANT: do NOT put the screws into the 2x4 where the holes are! You do not want the short screws to block the mounting holes.

NOTE: I made two sets for two French doors. You only need two slots per door.

NOTE: Once the slots are made,paint them and the barrier with outdoor paint. Otherwise, the weather will degrade the OSB.