The dimensions are clear and note that the blades or arms are "let in" to the post by half their thickness. The post and the blades were built of wood, the only metal being the bolts attaching the blades to the post. The blades are 1 1/2" thick and 8" wide.

Here is a clean example of a CNR version of the traditional crossbuck sign. Note that the lettering is only a partially "italic". We suspect that the trend to square lettering occurred sometime in the 1950's as this example shows. As was CNR's practice, the post is lettered with their "monogram".
Also note in this that the ends of the blades or arms are cut on the vertical (plumb cut) as in the plan.
Further development with time resulted in the blades being made of metal and the ends squared off at right angles instead of the plumb cut. The post was still made of wood. However, the blades were both mounted on the same side in contrast to the opposite faces of the post.
Here is an excerpt from a newspaper article from 1986 showing even further design development
For modelers of the steam and early diesel eras, it is essential to acquire or build models in accordance with the early standard plans. While a plastic version has been available from Tichy Train Group as part #8180, the blades are mounted on the same side of the post and the lettering is square to the blades. They do have the correct plumb cut on the blades. Our layout used these Tichy crossbucks for some time but eventually, we decided to build some in brass. This was due to the crossbucks being close to the layout edge and susceptible to being broken by errant hand and arm movements - usually when cleaning track.
We decided that correct lettering was in order since we were going to the trouble of building in brass. This would require custom decals. While we were at it why not get decals made up for the other signs along the right of way? So, many hours were spent going cross-eyed to compose a decal sheet in the correct sizes for crossbuck lettering, station signs, flanger signs, mileboard signs and many others. Negotiating with Mr. Bill Brillinger was a pleasure and here is his address for those who are interested in his services.
William J .A. Brillinger
Precision Design Co.
email: billy@pdc.ca
web site: www.pdc.ca
Call/Text: (204) 324-4807
An alternative to solid stock would be square tubing but the top would have to be filled with solder or putty or bondo. Mr. Thompson used square tubing. The blades could be cut from 0.015" sheet brass which is still made by K&S. It is thin enough that the blades could be cut with shears or even a pair of sharp sciccors. One more possibility is to use round stock for the post which was used by the prototype. We did a couple of signs with round posts of 3/32" (0.094") diameter brass rounds which show in the picture below on the left. Also shown is the Tichy crossbuck on the lower right.
In order to solder the blades to the post, a jig was made out of plywood and grooves cut into the board at the right angles and depths. A similar jig coud be built up of pieces of wood of suitable thickness. The angles were transferred directly from the drawings. Here is the jig we made to assemble the brass pieces for the crossbucks. Note that the blades were soldered together and then trimmed to length and angle afterward. The flux pen used is shown at the top.
Of course these crossbucks could be made of styrene components and a jig would still be a help to fastening the blades in place properly and again trimmed to length after gluing. the two vertical lines in the jig to left and right of the post are the cut lines for the plumb cut of the blades. A felt pen is used to transfer the line to the blades for the plumb cut.
This was a satisfying project to work with pleasing results.