Suitable mattress for a slatted base with 2 inch gaps

I am 5' 6" weighing just under 10st. My partner is 5' 10" and weighs 11 - 12st
We are both mainly side sleepers
We have a Warren Evans slatted double bed. The slats are 5 1/2" wide and 2" apart.
What mattress would you recommend please?