As title says

How do I mount/secure my 6x4 rear speakers,iirc in the g40 the rear parcel shelf supports were threaded so you were able to mount it easily but it seems gt's are poverty spec and they are just smooth

Sorry for my shit description but how have any of you guys done it?

If you've got the plain pegs hanging down, fabricate a strap across the back of the speaker. Otherwise, snap them off, chase the hole through with a drill, and screw them on.