After some deliberation I have decided to buy a small powered sub unit. Give I very rarely take rear passengers the best place for this to go in in the rear driver's footwell as this should give the least muffled sound although I suspect the unit will not go under the seat.

So the question is is how should this be mounted/fixed. I have no idea of what mountings are normally used for this type of thing and I have no idea which bits of the car are suitable and which are not suitable for screwing into.

Can anyone give any sensible pointers?

Well, the bass box should come with instructions, but a good tip is to wire the box up & position the box in various places in the cabin. You may find/think that original resting place may not be the best. Have/leave plenty length of wiring when choosing. Once you've chosen I would stick a couple of sheets of sound deadening material, then its a case of either brackets or screwing directly if there are holes on your bass box.

It's not really hard, the crucial bit is where it sounds best in the cabin for you. Hope this helps, PS have the box fully working before you secure it down by the way.

Within the cabin I can only think of the rear footwells it could go in. In the boot it would probably go in the cubby hole.

Good tip on the "see it working before you screw it down".