Hi Irongrave,
You’re welcome, but I’m sorry I don’t know what you mean by a gurney style bed. The Invacare beds are for sure aesthetically ugly, and are only available in twin extra-long as far as I have seen. Also, depending on your mattress, if you did use an Invacare bed then you might be well served to place a sheet of plywood under the mattress to isolate it from the spring support system of the Invacare bed. This is a link to the Invacare bed I was thinking about at the Walmart web site.
Regarding the HD polyfoam wedge, I don’t know how compressible or “soft” the foam is, but I imagine that if it is even a little bit “soft” then it could alter the performance of the mattress. Also, because from one end of the wedge to the other there is progressively more thickness of foam, it would progressively have more and more effect on the mattress from one end to the other. However, if you place a sheet of plywood over the foam wedge, this should isolate your mattress from the effect of any sagging or softness in the foam wedge.
Home Depot will cut down a 4x8 plywood sheet to your desired size for no extra charge. Unfortunately the cost of plywood is really high these days, (you can easily spend $60 on a 3/4 inch thick 4x8 sheet) so you have to balance cost with thickness. 3/4 inch is recommended for mattress support, but in my opinion that’s overkill and 3/8 is more than adequate to prevent excessive sagging as long as the plywood sheet is well supported. If it’s a relatively firm foam wedge that you’re dealing with, a sheet of lightweight MDF board might even do the trick to isolate the mattress from the foam wedge.
On the other hand, a 3/4 inch sheet of plywood under the mattress would be strong enough to be used all by its self to incline the mattress, if the plywood was lifted off the bed frame on the end and in the middle with some sort of lumber pieces cut to appropriate height.
The foam wedge you mentioned is 1 inch thick at one end and 7 inches thick at the other, for a total elevation of 6 inches over the entire length of the mattress. That seems like a fairly minimal degree of elevation. It would be nice to be able to adjust the amount of elevation in case more or less elevation is desired.
Spindle Mattress has some suggestions here for suppliers of robust bed frames. Perhaps those suppliers might give you some ideas, although when I glanced at them they seemed rather expensive.
A frame can certainly be strong enough to not require center supports, but this will make the frame more expensive because it will require more material.
One thing you want to be aware of is that on inexpensive metal frames, the legs can break off or can fold over when the frame is inclined, so you kind of have to judge for yourself if the legs and frame looks strong enough to support the progressively stronger lateral sideways force and induced frame twisting that is incurred as the frame is progressively inclined more and more. As for the legs in the middle of the frame, you would probably want to implement some kind of supports for them. Some 4 inch lengths of 2x4 and 1x3 boards stacked up to an appropriate height would provide for an adjustable-height support for the middle legs.
If your bed frame has wheels or can accept wheels, it turns out that you can buy adjustable height bed caster wheels. Meanwhile there are some so-called adjustable or stackable bed risers that allow for some limited amount of adjustment by stacking more or less of these risers together. Combining some risers with the adjustable height wheels would allow you to get exactly the height of riser you need, because the wheels can be fine tuned to exactly the needed height, while the plastic risers can give you some gross adjustments between different amounts of elevation.
Somewhat mid-level on the DIY difficulty spectrum would be to reinforce your existing metal bed frame with some lengths of 1x6 furring boards. Some 1x6 boards could stiffen the steel frame. The boards can be attached to the frame with self-tapping screws or bolts that are run through pilot holes drilled in the steel frame. While this could be done, it would seem to be a lot more simple to just prop up the existing legs of the bed frame with some sorts of risers, adjustable height wheels, and/or scrap pieces of lumber.
There should really be more inexpensive and versatile solutions on the market for inclining a bed or mattress, after all it’s a common need that a lot of people have.