As you are a sound engineer, you're probably aware of this, but I'll chuck it into the mix now, before you get stuck in and decide you don't like the result:
If you are doing conventional Blumlein stereo, you really don't want the speakers pointing at right angles to the wall behind your desk. The theory says you want an equilateral triangle, with the speaker axes along two of the sides, and your head at the opposite corner. In reality, you can vastly improve the image stability and depth perception, by crossing the speaker axes slightly in front of your head. Working close, I assume for editing, that would probably be around the tip of your nose, but possibly further away. It works because as you move your head, you come on-axis to the speaker furthest from you, and it relies on the fact that most tweeters have quite a directional polar pattern.
Your drawing has symmetry, but you probably want either asymmetry in each stand (a mirrored pair), or probably more useful, a disc at the top so you can swivel the speakers to suit.
The above technique works so well it's like alchemy. I remeber struggling to get stereo edits tidy - I grew up in the days of tape editing, and edits would swing horribly. Simply by crossing the axes slightly in front of my head, the image became a lot sharper, and you could hear what you were doing much better!
The other thing that might be worth doing is laminating the support and/or tying the two stands together with a horizontal piece (or pieces). Laminating will dampen the resonances, especially if you use dissimilar woods, and tying them together (out of sight behind the desk, and possibly at floor level) will greatly improve physical stability, but it will make it harder to set up. But see my final ccomment about the stability of the whole thing...
Regarding spikes, etc., you might consider adjustable ones, as these things will need to be levelled very well, in order for them to be stable, and I'd design the bases as triangles rather than rectangles, so that they can't wobble. Ties between the two sides will work against this however. Adjustable spikes are easy to make if you have a pillar drill or a lathe: a bolt plus a pronged tee nut (for the underside of the base), plus locknut. Hacksaw the head off and point the cut off end with a file in the drill or properly in a lathe. If you don't want a spike, use a coachbolt with a mushroom head. Set them up straight with a thick pile of old books on top, instead of your precious speakers!
Finally do what we used to in OB vans: strap the speakers to the stands! You'll kick yourself if a cable gets pulled and something on your desk goes right through a drive unit as the speaker heads south! I'd even consider fixing the stands to the back corners of the desk. My LS3/5As are probably worth more than everything else on my desk, and the desk itself, added together!
Hope that helps,
E.
PS there are some real loudspeaker experts on here, so you are in the right place -- I'm only a user, not a designer/maker!
