Thanks reserves. I was thinking of improvising with a waterproof mattress protector - the type that stops a young child wetting the bed. This would work especially well for the upstairs mattress. Here, the idea is to protect it from dampness in the bellows after unscheduled rain. (We'll have a roof topper for when it is very cold / wet but you never know when a heavy downpour might come).
The mattress protectors seem to come in standard bed sizes: single; small double; double etc. But, a standard (home) mattress generally has about 20cm depth and my memory of the Cali mattress is that it is a lot thinner (especially upstairs). So, I was wondering if a "single" size protector would fit over the mattress.
I already have one (that my 9 year old hasn't used for a few years now) so it would save quite a lot on the pricey Comfortz offering (even though the Comfortz product looks excellent quality).