I just place collars under each of the six legs. You can easily add 3" of headroom. Much more and the cover won't have enough overlap on the bed's side panels, etc. Also any longer and you'd need to make an actual extension for the legs else you won't have enough footing to hold the frame in the receivers (but you will eventually find a gap between the cover and sides as you raise the frame).
The collars work just fine if raising it a couple inches will help, though. I'd suggest a spot tack to retain the collars to the frame base. I lost one once due to the bed bouncing around while off road. Cheap and easy mod and it can be removed easily.
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