Almost anything that you could put on either side of your bench which is right at your chest level, could allow you to let the bar down and depending on how high you raise them, the bar might still be touching your chest but the full weight won't be on you, so you could wriggle out (unless you have a big gut).... I guess it depends how low you go with each rep, as you don't wanna be banging the bar on these things with every rep.
But you could almost use anything, as long as it's stable and secure and strong. As long as when the bar touches your chest, it's almost on these supports. Then you could let it down, and slide out backwards. Of course, that's an extreme measure.
The other alternative (if you're handy with stuff like that) is to weld the emergency rack things down a bit lower, just above chest level so you can still get the bar on there before you're at the sticking point.