Hello Garry -
Despite what I said, I do realize that it is largely a matter of convenience - and cost. Instead of cutting pieces of quality wood and then gluing it together for a base, it is much easier to just select a ready-made frame. In this particular case, besides it being too small, I think the style of it also gets in the way of your dio. The curved shape of the frame elements distracts the eye from your work, which should be the focus. I think it also presents a confusing message, because you have created this scene of northern Africa, but we naturally associate the frame with something else, and for me this creates the conflict. One other thing that this does: you work to make your diorama three-dimensional, but such a frame suggests that it surrounds a 2D work of some sort, so it "grounds" the dio and decreases it's ability to achieve height. At a minimum, I'd encourage getting a frame that is large enough for what it contains, and has a plainer and more narrow profile - you don't want it competing for attention against your work.
I have another, more general issue. I'm not understanding why these skilled craftspeople, who spend many hours building and modifying kits, painting if not rearranging figures, and working hard to provide just the right weathering and finishing effects, are then willing to be lazy and place their work in what is often a cheap, common frame. To my view, the base is better the closer it gets to being invisible, because the extent to which it is visible is also how much potential it has to distract from your work/dio.
Another possibility is to create your own base, but not trying to make it a picture frame. Perhaps use other materials besides finished wood, maybe trying to create a base that supports what you are trying to achieve with the dio, instead of distracting the viewer from it. Try to select materials and a design that complement the dio. It may not even need to be solid. If you are interested in presenting the dio as a small slice of a different reality, try to make a base that lets it "float in space" to some degree, so that it can further isolate from everything else around it - either where you live, or from all the other dios next to it at a show. Try unusual materials, but that will work with the dio.
Let me suggest something else - a different approach to placing the scene on the base. I found this photo about a week ago among many images of a European show.

By using this technique, the modeler has assured that his scene doesn't run the risk of being defined by the rectangular edges of his base. By doing this, he is controlling where your eyes look, or perhaps more accurately, where they don't look. Defining the outer limits of the dio by the extent of the scenery material, he makes certain that the scene ends before the hard, straight lines of the actual base. This actually reminded me of photographs I had seen years ago of a most unique model railroad designed to travel to exhibitions.

By painting all areas where there is no track a flat black, the builders have clearly defined where they want people to look, and the layout stands out by contrast. One note here is that they have complete control of the lighting, while you're not going to always have that for your dio - especially if you bring it to a show, unless it's self-contained in some sort of shadow-box display.
Charles