Mine is 3'x8' supported by 2x4 framing having the back length beam attached to my garage wall. The left and right of the frame is supported by base kitchen cabinets, leveled out which provide oodles of storage 3/4" sacrificial ply screwed on top of the cabinet supported frame which I replace every couple years due to it getting yucky,.... not warping, etc.. On that I build on inverted 2 x 4 ceiling tiles which is great for holding pins, etc

A metal frame is nice but not needed in my opinion. My shop is a no A/C or heat garage in west (2 mi from Gulf) central florida. No issues with warping or delamination of my ply top.

Unless your buying solid wood slab exterior doors, those intended for indoor A/C use will warp as they are basically hollow with thin wood strips internally with thin ply skins on top.