Beyond the reusing the original screw terminals, mounting it to some L-brackets is the next most obvious option, and it's relatively straightforward:
•Place a L-bracket atop the motherboard, inside the footprint of the shell
•Place the shell atop the motherboard - if need be, tape the shell in place.
•Lift the shell and motherboard, and tilt both towards the floor - the L-bracket should slide up against the shell wall.
•Gently return the motherboard and shell to the table, and carefully remove the shell. The L-bracket should remain in the position it occupied whilst the shell was in-situ.
•Draw around the footprint of the bracket
•(If the L-bracket has already been drilled for bolts/pop-rivets, use these to drill the corresponding holes through the motherboard).
•Repeat the previous steps 2-4 times (in honesty though, you may only need two L-brackets due to the smaller, lighter nature of the Spirit pack shells.
Alternately you could draw around the shell whilst it is sat on the motherboard, measure the thickness of the shell wall, and determine where the bracket should fit that way.
It'll be a lot easier to do the L-brackets on the Spirit Pack compared to a full-scale fibreglass shell.