Well my kid overgrew his standard 120×60 cm bed and I was think what to do – I already made a bed out of pallets for me and the misses so it just landed on me in sec: hey, standard pallet size is 120×80, I have a spare mattress, lets give it a try…After few hours of sanding an painting and leveling the original bed base, I attached some old wheels and got it moving…
Now my kid rumbles around a 120×120 bed like a King! I still have some minor tweaks to do (mainly to add another thin mattress to cover up this “joint” where two mattresses meat) but general idea is here…Hope you like it – I would want to believe that this one is the original idea – you tell me 
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