Hi K-Lo.
This depends. How thick of a mattress are you looking to put on this v-berth? Ultimately, all the same rules that apply to a standard sized mattress are going to apply here as well (factoring in your BMI, sleeping positions, etc). A platform surface will create a firmer experience on the mattress. If, for example, you are considering a 6" mattress for your v-berth, you may wish to consider 3" of a softer ILD atop 3" of a medium ILD, letting the hard surface of the v-berth act as the firm “base.” This would allow you to sink into the soft layers but be met by a supportive (but not overly hard) medium transition layer to act as a buffer between you and the platform on which your mattress lies.
When done with patience, precision, and the right equipment (an electric carving knife is a great option for cutting latex) it is likely you could cut the layers down yourself. That said, our Trusted Member DIY Natural Bedding will do custom latex cuts, so that may be an option or venue worth looking into as well.
You can read a little bit more about cutting your own latex in this forum topic.
I hope this helps!
NikkiTMU