Screwing down a ute floor

As a follow-up to my thread a few weeks (months?) ago about the sheet for my ute tray build;
I have the sheet waiting to go on. 2mm zinc in two separate pieces. As mentioned in my other post, my plan was to glue the sheet to the frame with sikaflex or similar, however I have ended up with so much stuff mounted under the tray that I will inevitably want to remove the sheet for maintenance at some stage. I have a full-length trundle drawer that carries 2 spare wheels, the number plate and reverse camera are mounted on the back of the drawer with the wiring carried in that roller-chain cable track stuff, I have installed remote-controlled air spring suspension and all the solenoids and gadgetry are under the tray, I have spring tray-to-chassis mounts- so yes plenty that will need to be accessible so glueing the floor sheets on is no longer a goer.
I'm now looking at screwing the sheets to the frame and seeking some advice about this. As stated, the sheet thickness is 2mm. The frame members are 40x40x2.5 SHS. I plan to lay some 2 or 3mm insertion rubber on the cross members before the sheet (open to thoughts on this).
I have some M6 countersunk capscrews (not enough for the job, but enough to measure) and the head is 3.5mm deep so would have to be countersunk beyond the sheet thickness to avoid being proud of the surface. I'm thinking possibly M5 countersunk screws but have none to measure the head depth. If I go this way, will the 2.5mm wall of the SHS be enough thickness to hold thew floor if I tap the thread into it, or should I use rivnuts?
To clarify what I am asking:
M6 or M5 countersunk screws? Rivnuts or drill & tap?

Thanks.


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