We have a Cali-Cozy internal 'topper'.
https://www.comfortzleisure.net/index.php?route=product/product&product_id=102
Admittedly it's more expensive than an external topper but is zero faff because, once fitted, it is left in situ.
Advantages:
- It's always in place (raises and lowers with the roof)
- Keeps the interior of the van much warmer
- Cuts out the wind (and the rain) that blows through the vents but still permits ventilation
- Reduces sunlight through the canvas so better for sleeping during summer months
- Waterproof, so roof can be closed wet without making the upper mattress damp
- The bellows side and front windows can still be opened
Thoroughly recommended.