I don't think that the runners will bend. Check what's under there now. You probably don't have the tapered wooden spacers, but if you do, then take them out and start from there. If you find that the runners need some mid support, either make, or get made some new wooden wedges - (just cut a suitable length on a diagonal), or a nylon block could be cut and fixed with double sided tape mid runner. Funnily enough, although high density poly wouldn't be hard enough, rubber sheet would be, and could be laminated into a wedge.
