Zinus sells a range of easy-to-assemble bed frames and mattresses.

Zinus

got the queen size in super firm and it's honestly great. i saw concerns about fiberglass (iykyk) so we wrapped ours in an extra sheet layer to keep it clean. super comfortable and really affordable (think we got ours black friday)

I just purchased one of their super easy-to-assemble bed frames and, even with a bit of issue, had the entire frame put together in fifteen minutes. The one I got, in particular, had a very pretty case to wrap around the metal frame to make it look like it was upholstered. Overall, a great purchase. 

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