Yes, Ciive's suggestion of laminations to produce a slot is a good one, and then to have more than one slot. You could even make a four way toolpost..
My fourway Rear Toolpost was laminations held togerther by three long Allen capscrews, with the fourth hole used for a dowel..
How do you ensure that the plates are square?
If you have a four jaw chuck, hold the plates in it and face each edge. Intermittent cut (Ah Chicken and Egg situation! So make a rough toolpost first of all, so that you can get started )
Having faced one side, use that as a datum and set it against the Chuck or faceplate, so that you will then have two faces parallel. Then use a square to set those faces square to the chuck or faceplate to machine the other two sides.
Once the , presumably, three laminations have been fixed together, so that the outer edges are in line,, it can be mounted and set so that it is square across the lathe. (One face against the Chuck or preferably, Faceplate, so that the vacant fourth hole can be used a jig to spot through into the Cross Slide, ready to be reamed for a long removable dowel)
Once the Toolpost has been unclamped, the dowel can be lifted, and the post indexed round for the dowel to align it for the next tool in the post.
As TV used to say about Bionic Man; we have the means!
HTH.
Howard