Hi,
We have a 'Rainbow Screens' pop top wrap for our LWB. Got it earlier on in the year and have used it twice. it does make a difference. Especially with the heating on. We have a diesel heater and they are quite over powered for the vans but the standard pop top canvas does struggle to retain the heat - so you would have to have the thing on high to achieve any kind of warmth up top, especially with the bed down. The cover solves this problem with the added benefit of blocking out most of light that come through the canvas and lessoning the sounds from outside the van.
They are not cheap at around £300 but worth it for the chillier months.
Cz.