We setup a silver tarp over the main camper tent, with the fully enclosed annex left exposed.
When we were away in Jan, on the hot days (when we couldn't go to the beach as my daughter had just broken her arm), it was too hot to do anything in the annex, but in comparison the main tent felt like we had an air conditioner on. Cool enough for most of the fam to get a good afternoon siesta in

We bought extra poles and spreader bars to make up a frame, and replaced the sprigots on the poles between the annex and main tent with 150mm ones. I then cut a thread onto the end of these, put on a coupling bolt and a lock nut, and added about another 150mm of threaded rod to the top of each pole. The silver tarp was then screwed on the top of that (with wing nuts and washers). In all, the silver taps sits approx 200mm off the main canvas. Had some wild wind whilst we were away, and just threw an extra rope over the middle of the taps to help secure it, but in reality is was not really needed. Added a bit of noise as the tarp blew around, but it actually wasn't all that bad.
Looking to get the biggest spreader bars (7m??) and a bigger tarp to cover both the camper and the annex in the future, as we reckon it really helps out. Keeps it darker in the morning as well for those well deserved sleep ins too.
Cheers
whitedg