So the question is, what would be the best "insulator" to put under the mattress pad? Don't want something that makes crinkly noise or that is uncomfortable but just a tad of insulation. We don't want to have the bed much higher, as Ruth doesn't like to have to take a running leap to get into bed.

First I suggest lining your plywood base and surrounding walls, below the mattress height, with the thin foam-backed reflective aluminum sheeting. Also a thin down mattress topper (not a comforter) does not add much height or weight but adds warmth. Best, and most expensive, would be a memory-foam mattress.

I am wondering about bed setups. We have an '74 Eleganza SE with a plywood platform and a regular double bed mattress from JC Penny on top for the plywood. We don't venture out when it is all that cold (we are California folks) but when we do we notice that when it is freezing out we feel cold coming through the mattress. We have plenty of blankets and down comforters, etc. and so are warm... on top. But we both feel cold or at least too cool under us.

So the question is, what would be the best "insulator" to put under the mattress pad? Don't want something that makes crinkly noise or that is uncomfortable but just a tad of insulation. We don't want to have the bed much higher, as Ruth doesn't like to have to take a running leap to get into bed.