Hi frhughes3.
Welcome to our Mattress Forum. 
A DIY can be quite rewarding when approached with a sense of adventure! Because your current mattress is too firm, you could do a sort of semi-DIY in which you add the appropriate comfort layer in the form of a topper.
The mattress you tested sounds interesting! While I’m unsure where to find a source for the 3 LB HR Cold Foam you mentioned, there is this thread on soft DIYs for soft sleepers that may have some useful information for you in the DIY aspect. Hopefully someone else here in the forum can chime in with respect that the specific base foam.
NikkiTMU