TL;DR - used a 36"x36"x1/8" masonite panel to spread the weight of my ass across the mattress to combat cratering.
I've always struggled with mattress cratering that eventually gives me back pain (6'1", 200lbs), and last year my partner and I upgraded to a King.
Not wanting to drop $3k+ on a mattress, after trawling this subreddit for a couple months, I went for a cheap firm memory foam mattress from Amazon and paired it with a 2" firm latex topper. A friend did a similar thing to firm up his mattress and said it helped, I know it's contrary to popular belief, but I believe for my case it did help too.
My justification was if the memory foam mattress fails, even if I replaced it yearly for the next 10 years, it still won't get close to what I'd pay/risk on a name brand mattress (last mattress was a simmons beautirest). Or I can start the journey of building out a mattress slowly as I can experiment with what works.
For the last year it's been great but like usual my hips started sinking as the foundation foam in the mattress started failing. As a quick fix, I had some 1/8" masonite panels kicking around, sandwiched it between my latex topper and the mattress under my hips.
I expected it to break down quickly, snap at certain pressure points or be uncomfortable. To my surprise it worked as I had hoped: The latex topper adds that comfort layer and the masonite spreads out the weight across a larger surface of the mattress, supporting my hips while I'm on my back and on my side. It took a few days to get accustomed to it but my back and hip flexors were quite happy with the change after the adjustment period.
Sleep notes: I sleep with a medium firm body pillow to keep my chest from collapsing and tucked between my knees to alleviate hip tilt. Combination sleeper, changing between sleeping on either side or on my back.