Replacement mattress for a wooden slatted bed frame.

I have a 15 year old pocket spring Silentnight firm mattress on my adjustable slatted bed base. I find the mattress too firm and require to find something more suitable for my creaky joints. I have tried out a wool topper and padded mattress protector combination with limited improvement. I am 5' 2" tall, weigh 9 stone, sleep on my front and do not like memory foam or similar. The internal measurements of the base are 63.5" X 78.8" I plan to use your mdf boards solution to modify the base. I have a Restus Cashmere medium mattress on the spare bed, which I find very comfortable. Please advise.