The belts shown have open clips so they can be attached and easily removed. You don't need any special fixings but they wuill need to be connected tio eye bolts. That will enable the belts to be left in-situ or taken out when not needed. There are, however, some regualtions on eye-bolts.
Eye bolts for the securing of harnesses (as I seem to recall the DVLAS (uk) saying) need to be secured to a captive nut. For the lap belt this means you can remove one of the bolts on the door side of the floor mounting and replace it with en eye-bolt. For the centre you will need to attach it by replacing the bolt that hold the femaile end of the seat-belt to the car/seat.
For the shoulder harnesses these will need to be attached roughly at the rear of the lower swab of the rear seat. In front of the rear seats and the angle is too acute. Above the rear seats (i.e. the parcel shelf) is not regarded as structural and will not be permitted. You can not simply attach the eye bolts through a hole in the brace member with a nut. Instead yyou will need to secure a captive nut first and then aattach through a hole to the same captive nut.
Hope this isn't too confusing.
John