This is a DIY situation for sure. Almost no transformer making tools are in reach of the small guy, other than winders. Most transformer manufacturers make their own tooling for stuff like this.
If it were me, I'd try woodworking corner clamps and a couple of bench clamps to hold them around the lams. Like in this picture, but with the squares cut down to size and turned towards each other, holding the lams in between. Then use the bench clamps to hold the opposite sides of each angle tight enough for the job.