Updated Pics for shelters with different frames & wall panels  made on platform

Bare platform

 9 ft long 'H' shape concrete fence posts as base. NOTE: Reinforced concrete H beams are used as they are strong,  cheap , £10, fire resistant & heavy, so when base is attached to them , it will help stop structure being blown over, by weighting base

 Fence posts may need to be strengthened with purlins or drywall lining

CD Panel floor with 2 floor beams ( 3 needed for heavier weight loads on floor)
  

 CD Panel floor with 2 floor beams running  the other way, with edges resting in H recess of beam

An advantage of the H beam is that a 45-50mm wide paenl will fit in the groove

Option B. Frame made from 48mm scaffolding poles & metalclad beams

Close up of frame made from 48mm scaffolding poles & metalclad beams, a door edge strip is on outside to insualte metalclad steel from outside to prevent thermal bridge