http://shop.vendio.com/wfutons/item/922628614/index.html

This is World of Futon website, have you seen these frames?  I know the slats are far apart, but the price is reasonable and it is adjustable, and made in the USA of natural wood.  I think you could modify it somehow with a grate of some kind and loop in in with the electrial tie down clips.  Just grasping at straws trying to find a simple solution.

 

Here it is at Amazon with reviews.  They also have just regular frames, not the adjustable futon like I linked.

Here is the regular bed frame at Amazon.  It got great reviews.  Does anyone have the frames from World of Futons?

What really is a futon mattress?  Does it have springs?  I read about them and really don't understand what they are.

A futon mattress is generally about 4" to 6" cotton or cotton/wool batting with a cover. In my experience they tend to compress fairly quickly, the cheaper ones go very quickly.

I'm not sure what you're using it for? If it's for a cotton-batting type futon it looks like it would be okay. We have one and it serves well as a couch.

Looks like a decent frame and I don't think the space between the slats is a problem for a futon, but it could be. Ours is a little closer together.

One thing I don't like about ours and this is the same way: the seat slants back at an angle so it's not very comfortable to lay in as you're not laying flat. Of course when it folds down, it's flat; but not when it's used as a couch.