If you're making a smoke shelf baffle plate, you only set the flat plate on two bricks on their side edges at front edge of plate. The drawing below shows brick and plate.
If you're building your own angle iron frame, like pictured in above post #7 link; build frame to fit bricks or cut bricks to fit frame. The "smoke shelf baffle" single plate is simple and takes minutes to install.
Measure across the inside of stove wall to wall (above brick) and make or purchase plate about 1/2" shorter so it fits easily across stove.
The depth (front to back) size varies with pitch. If you're putting the plate flat horizontal, measure from back to about where the outer shell stops (where the front of the Insert sticks out of the fireplace) Make sure the baffle covers the outlet, so no rising exhaust goes straight up the outlet without going around the plate. The steeper the angle, the longer the plate needs to be. You can draw a line where it will go with chalk on the inside (or outside if not installed) and measure the length of line for an approximate.
Depending on height needed at front, you can lay a full brick on it's long edge on top of the side brick retainers where front of plate will be. With plate on an angle, it will set on the corner of brick. Or if you need it higher, stand the brick up height wise and use that to set the front edge on. As you move the side bricks forward, the plate will lower in the front. Pushing them toward the rear will raise the front of plate.
These are one piece brick retainers down the side to set an upper brick on against wall.
You can set the plate directly on the rear retainers, or put a brick sideways across back, on edge to raise plate for better loading area under plate.
Some stoves will have small lengths of angle iron like tabs for each brick.......... In that case you have to set brick between tabs, possibly cutting a brick. (or cut corner on an angle to sit plate on) They cut easily by scoring with masonry blade and snapping down the score line.
Lots of pictures of what you have are needed for details of the way you need to do it.
I'll draw
baffle in red and
brick in blue on the side of a stove drawing. (click on drawing to enlarge) The blue line is top of brick retainer inside stove. Do the same with chalk measuring where the brick retainer would be through the stove wall. (transpose blue and red line on exterior of stove with chalk to get plate line length)
The plate can stop anywhere as long as it's ahead of the vent opening. The farther forward, the more heat directed to the Insert top that sticks out of the hearth. Not a factor if using a blower. This moves the hottest area that WAS at outlet to above front plate edge.
These are measurements of the full size insert.
As you can see, sliding the added upper brick drawn in blue raises the plate when slid toward the rear, and lowers plate when slid toward front.
Adjust plate height at front to the smoke space required. This cannot be smaller than outlet vent opening square inches. (50)
As an example, a stove measuring 25 inches across the inside would need 25 X 2 inch space, or 50 square inches.
