I am a bit with Somerford here. I would fit a short pair of centre legs (or even an extra couple of pairs). These things always need levelling up and four legs is hard enough, let alone six, so I would accept the need for 'shimming' and leave the middle ones short. Add at least one through-bolt, with large washers, on each corner for longevity.
I like the hand spaces but the boxes always seem to be welded on when I want to remove them and I never have anywhere to slip in a wedge (or hive tool) on mine.
So a good idea which I will follow up. Good for slipping tie-down straps through when moving colonies, too. So d*mned simple, I wonder why I never thought of it when I made my hive stands! Or since, for that matter!
I don't like double stands so all mine are singles - doubles won't fit my car, are too heavy and any other reasons not to have them, but that looks fine (more stable than singles is an advantage), will do the job and, you will doubtless find out that suitable timber will now surface for free, of course! So job well done. You might need a small fillet to fill that little gap from landing board to hive front.
For the budding DIYers, I used 50mm square legs and cut rebates on two sides for the timbers to bear on, so screws were fine as fixings (not bearing a load) and the footprint was about 450mm, so only about 10mm smaller than the hive, which sits exactly over the stand. Much lighter construction than Hawklord's and easier to knock up once the leg rebates were done. Mine are 300mm high (made before I changed to 14 x 12) and are not far off a good compromise for stability and comfort with that size box. They were just a tad too low, with a standard brood box, for manipulations - I have a bad back at times.
They nearly always benefit from standing on some pieces of scrap wood. The original two I had when I first started have rotted at the feet and have been pensioned off as they were of the more 'simple construction', have slowly become less serviceable without some serious maintenance, had that slightly less wide stance at the feet (so slightly less stable), and were only about 250mm high (so even lower for brood frame manipulations).
If I made them any taller I think I would seriously consider increasing the footdrint for more stability.
Regards, RAB