There are two complete shelves (with the clips) on ebay (one is open to offers, if you could get it at £100). I know its a lot but these aren't available on their own.
https://www.ebay.co.uk/itm/09-15-BMW-Z4-E89-REAR-BOOT-TRIM-PANEL-COVER-PANEL-7341663/293718799152?hash=item4463013330%3Ag%3Aa4wAAOSwLSBfUKye&LH_ItemCondition=3000
https://www.ebay.co.uk/itm/BMW-Z4-E89-Convertible-2009-16-Roadster-Rear-Boot-Trunk-Lid-Trim-Panel-Cover-Net/324164968927?hash=item4b79bce1df%3Ag%3AbiQAAOSwShheu-70&LH_ItemCondition=3000
If you have the broken ones its an easy repair