Not sure if these are the same ones, but this is a York Modelmaking magnetic clamp on a Coopercraft wagon I'm building (first wagon I've built for a couple of decades, or more!). I put a piece of card packing on the end to get it square. I actually used two clamps on each corner while sticking them, attached top and bottom, ran some solvent down the middle of the side, then slid each clamp down in turn and applied solvent at the top and bottom. I'm still getting used to using them, as I've only had them a few days, so will hopefully refine my methods in time.
This shows the clamps more clearly. This is a Peco wagon I bought not quite finished on eBay, whose end fell out when I was about to fit the buffers! The reattached joins are slightly better than they look here, as I was still working on it!