Camper cosi used for first time

[daisyduke]

used last week for first time. Set up/ taking it down was fairly easy even on my own with use of small folding steps. I dont have a bike rack on the back so needed to just let the strap tuck in under and be trapped when the back boot door closed( as per instructions). Problem was had some very heavy rain and it seeped in via the fabric clip/strap.This meant wet rear parcel shelf mattress and bedding
Does anyone with one know of a better way to secure the rear section when you dont have a bike rack to clip it too? what simple thing am i missing to make this fool proof? having done it as per instructions they dont cover heavy rain issue!

it was great to use and made the van super snuggly but was a pain to get folded up and back in the bag but i am sure i'll get better at that if use it more.