[Boatanchors] Plugging Chassis Holes

[L L bahr]

How do you guys PLUG steel chassis holes when refurbishing an old radio?  I have been epoxying a backer plate inside a chassis and then filling in on top with steel epoxy, then sanding and painting.  This seems to work well unless you have obstructions underneath and can live with a visible plate inside the chassis. 

Is there a good way to do this without a backer plate? 

Lee, w0vt