It's been a few years since , I opened mine up , I do remember there were clips on all four corners.
Here's what it looks like from the rear . photo below
I'll be at the coach tomorrow and see if a close look will jog my memory.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.