Geoff,
I'm glad that arrangement will work for you as well as it has worked for me.
The header tank sits in the bay with a bit of Velcro on the bottom to hold it down on top of the pump. The pump is attached to the fuselage floor with Velcro as well. Simply coat the fiberglass with some 5 minute epoxy glue to make it smooth, and then adhesive backed Vecro will adhere nicely.
You could also apply some 5 minute epoxy to the end of each inlet trunk and use Velcro to attach the header tank.

Harley Condra
BVM REP
JetCat REP