Some time ago I came across a picture of a mobile observation tower in the Airfix guide to 8th Army in the Desert (number 20). I don't think anyone makes one in 15mm, so I had a go myself.
The tower is a little wider than the one in the picture I have (for ease of construction) and it's based on a Morris AA tractor (with the ammo boxes removed and some sides added) not the 30cwt(?) in the picture, but there is no mistaking what it is and it will make a good, and different, FOO stand.
The tower is made out of wire pins (the uprights), and thin card (red stuff) soaked with superglue. The metal bit at the top is a turret lid and this was the key to construction: I pushed 4 holes it in a 'square' pattern and glued the pins in these first, the other end of the pins were glued into holes in the resin casting - square, no-wobble solid from the start.
The figure is the upper half of a British officer and the legs of a German motorcyclist. The AA tractor and figure are FoW.
If you are wondering why they built them. The Western Desert is dead flat in most places and there are no tall buildings (not many buildings actually); if you wanted 'high ground' to observe from you had to take it with you.