Just table it out, so you have a nice waist high platform to slide all your junk into and out from. The costs to change any of that structure for storage would be crazy and you would end up doing damage to ceiling etc.
Why don't you try and make use of the space above the box trusses? The timber 4" x 1" is bracing and is important. If you were to make a raised platform, you could plyboard this section and do away with the top bracings as long as the ply is secured to the girder truss.
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