I have looked at the thick memory foam ones but not sure if it will then make the bed hot and be hard to move about, how thick are the AFE ones and how do you find them.
The thickness varies depending which one you buy, we have bought the thicker more dense versions of VF52 memory foam, which cost a bit more. The VF52 60mm is what we have and it works well for us. We place ours on good quality very firm inner spring mattresses and therefore you do not sink too deep, I do not know but I imagine over a low density foam mattress you would sink a lot and it may be hot. However, our experience is that they are snug in winter, but not too hot in summer.
We live in the hills East of Queanbeyan at 800m and have very cold winters (-5C common overnight) and very hot summers (35C days not uncommon). Our house is well designed and cools down most nights, but not all nights, if in a heatwave as per January last year it does not cool much. So we do have some very hot nights and have not had heat problem with our memory foam overlays. We have had a similar experience of hot nights in our caravan during our travels over the last three years.
There are many qualities and grades of foam, the cheapest are the least dense and do not provide a lot of cushioning and behave very differently to the more expensive high density foams, if you go to Clark rubber you can see the different densities of foam on sale next to each other. Some years ago when we were younger and tougher we replaced the foam in our swags with a much higher density and more expensive foam from Clark rubber and that extended the life of our swags for a few more years. They still work OK, but we have got older and softer and now sleep in a bed. The foam on the link page, from AFE is an expensive grade of foam, more expensive than Clark Rubber, but a lot cheaper then Tempur (which is very overpriced, but very good quality). The foam on the page of the link is memory foam which is very similar to the Tempur product, but at more realistic prices.
http://www.afe.com.au/Memory_Foam_Mattress_Overlay.htm
It is really up to you what you buy, but you do get what you pay for, especially if you do your research on a product. The AFE stuff is not cheap, but works well for us. Again we have no link with AFD, just have used their product and like their made to measure service for caravans and are happy customers.