I bought a new roof top tent of eBay a while ago. It was at the $700 mark. It looks just like the ARBs or Ironmans; I've even checked them out on expos to see if details are different but of course had no chance to set them up or pack them.
We've slept in ours for around 13 weeks by now all added up. Here are some of my DIY improvements:
The main thing is packing it up. The cover is just that little bit too small. OK, I do want to have my bedding all packed up in there for convenience and space. The biggest change was to extend the cover with some decent tarp. Took about a day to do, a sewing machine and a good needle or two.
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Then the straps which hold the cover (other than the zipper) were always to short so a bit of string first and then a $1 shackle extended those nicely. The shackle lets the other strap slide through better than the string as well.
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The tie downs on the side (which help to compact the closed tent) were always a pain and started falling apart. Velcro doesn't work that well under a pull neither. I replaced them with the cheapest real tie downs I could get. Works brilliant.
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It's always good to have some lubrication for zippers which are exposed to dust. Not really DIY but keeps things going.
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Last but not least I've put a LED light in the tent. I had a bit left over from the awning and boot LED-job so that was for free. I also had some clear hose lying around which I sanded to make it distribute the light better (LEDs are quite focused). Bit's of cable and a small Anderson plug to finish it off. Total cost $5 for the Anderson. Pro tip: make the cable which runs up from your car long enough to have its lowest point outside your window or else you'll funnel water into your car if it rains.
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What have YOU done to your roof tent? What experience do you have with your tent? Are there similar issues with the brand tents?