Well my first thought after looking at it would be to do it like the existing floor access panels;
- Cut away carpet where access is needed
- Trim carpet back at all corners and fold back
- Cut opening in comp floor
- Route a recessed lip all the way around opening
- Lay carpet back down and adhese around opening edges
- Cut aluminum plate to fit opening
- Drill holes into plate to secure to lip
- Cover plate with carpet and secure
Another method SC could do would be to manufacture a dam from their molds that could be pushed up from the locker into the void and sealed using asphalt roofing sealer. This could be done, i've seen trickier done before.