Hi. While it would be great to have this camper, assuming Volvo engineered a redesigned roof structure around this hole in the roof, just cutting the opening would IMHO be disastrous.

To install the camper top, you'll have to cut through several transverse roof structures that provide stiffness to (1) support the roof against a "rollover" collapse, (2) dissipate energy across the top (together with the bottom) to protect against side impact, and (3) contribute to chassis torsional rigidity. Consider the size of a Volvo's sunroof (on cars so equipped) -- they're small, to fit between those critical cross-pieces. This camper top's installation has to require cutting and removing them.

Just as you (expressed) fear, I would agree that it would severely reduce the structural rigidity of the roof, and therefore the whole body. Structural rigidity in the body is crucial, because the car is a unibody, and it relies heavily on the roof and A-D pillars to compensate for all the glass area. I believe that this kind of job would invite flex, groans and rattles in the body, and can even affect handling (because of chassis flex).

To prevent these drawbacks, you would have to fabricate a rigid box structure around the opening and weld the transverse braces to it. That would be very difficult, especially if you want to work with light-weight alloys.

Sorry.