Hi Mightyh,
[quote]My son is currently sleeping on the innerspring-over-wire-frame-within-metal-rim set up, so I guess I can assume the metal frame will hold an alternate system? I don’t know how I would otherwise determine the strength of the frame….
I can’t see any way to secure the slats/foundation to the metal rim. The current wire frame just sits inside it.[/quote]
Gluing velcro to the frame and to the slats would help prevent them from shifting.
If your frame is a standard size that is the same as a typical steel bedframe (something like these) with a lip on the outside then a standard size foundation in a low profile or a bunkie board or something like this that is the right size to fit snugly inside the metal lip should work well. There is more about foundations and other support systems in the foundation post here.
A foam mattress (such as latex or a latex/polyfoam hybrid) will generally do best with a rigid, non flexing, and evenly supportive support system underneath it. As far as the strength of the bunkbed frame itself and it’s ability to hold the mattress, your support system, and your son, unless you know the weight that it is rated for you would need to use “best judgement” based on the quality and the strength of its construction.
Phoenix