Constructing a DIY memory foam mattress requires a high resiliency foam (known as HR foam) with a density rating of 2.5 - 3lb/ft³ with an indentation load deflection of 26lbs or 31lbs according to Healthy Foundations, a memory foam specialist.
You can follow the materials formula and resources under the cut to create your own budget Tempur-pedic memory foam mattress...
The Nobot made his own memory foam mattress using the following materials from wholesale memory foam dealers:
From top to the bottom layer:
Total price, shipped, was less than $450.
As you can see, memory foam density is a highly subjective choice, as some may prefer a higher density, while another may prefer a cushier ride. Your best bet is to contact a foam dealer for small square samples to get an idea of each density rating and create a mattress accordingly. And if you don't like it, you can always use the memory foam for other projects.
*we'd like to note we dropped by our local Target and noticed starting Monday all the way through Saturday, Target is having a sale on all their memory foam products.