Yep, it's good stuff
![[:Y:]](./images/smilies/smiley20.gif "Thumbs Up")
Feels like cheating. Totally reusable, too. And it carves nicely, if you need to make any modifications after it's cooled.This is a good way to make a bridge caul if you use different shapes all the time like I do, and especially if the bridge thickness is tapered toward the back edge, which unbalances the clamping pressure (makes it want to slide forward). The plastic will form to grip the bridge pin holes so it can't slip.
And you don't necessarily need a lot of plastic. Rather than making an entire caul out of it, you can use a flat or roughly shaped wood block, and just enough plastic to fill the gaps between that and the target object.