Hi Drumheller,
I think the reason for their suggestion for smaller gaps is probably because the mattress is quite thin and you may feel the slats if they were further apart and there may also be some risk of the mattress “bending” into the slats.
If you don’t have any other “risk factors” then it may be worth considering a solid surface as well since I would call that a “compelling reason”. I have an adjustable bed which has a solid surface for example and for me the benefits of an adjustable bed was worth the “tradeoff” of a little higher risk with ventilation.
Post #4 here includes several slatted bunkie boards and one of them has 1.5" gaps. The one from Ivy Organics also uses organic cotton and wool so it’s more costly and may also be costly to ship but the flobeds slat conversions are very economical.
Yes … that’s what they are designed for and they would be fine. You could also use the bedrug or the slat conversion over a solid surface to increase ventilation under the mattress.
Phoenix