PSB Image B15 -
PSB Speakers - Image B15 Monitor
Size ........... (W x H x D)
Millimeters ... 178 x 343 x 248 mm
Inches ........ 7 x 13 1/2 x 9 3/4"
Placing the speaker on 500mm stands (~20") will place the tweeter at about 32" (813mm). That's pretty good, depending on how far away you are from the speaker. Though perhaps 36" or 38" might be better.
(EDITED: that 36" or 38" total distance, not the stands. The stands would be 24" (600mm) or 26" (660mm) tall. Still if you sit 8ft to 10ft from the speakers, even 32" should be fine. The tweeter fans out at about ±0.3 x distance. So, if you are 10ft from the speaker, the sound has fanned 3ft to the left and right of the center line. [actually the factor is 0.268 x D])
As to adding sand, you want some type of door in the stand so you can add and remove the sand as needed. You could also fill the unused portion of the stand with insulation or foam, to prevent it from acting like a pipe on a pipe organ.
Also, you need to make sure the stands are completely sealed so you don't leak sand, and are braced to take the weight of the sand. This is no big deal those, these are just minor design details.
Overall, if you have the height you want, and the tops and bottoms are large enough to accommodate the speakers and secure the stability of the base, you are golden. Beyond that it is just common sense in the design details.
Personally, rather than speaker terminals on the back of the stand, I would just cut a notch into the back of the cabinet, then cover that notch with a board. In a sense, simply make a channel down the back of the stand to contain the speaker wire. Again, just a minor personal design detail. I'm sure the minor details will all work themselves out in the final design.
Next, do you have woodworking tools? Primarily access to a table saw. If not, you can probably find a cabinet maker who will cut-to-size for you, and then you can take care of the assembly and finishing yourself.
Also, I'm suspecting that
KobeoneHD2 might have used standard sized lumber, and merely cut them to length. Consider that for your project. Getting a miter box to accurately and squarely cut wood to length is relatively easy and cheap. Even if you have someone do it for you, it will still be cheap since the cuts will be relatively simple and very few.
Remember when working with MDF, you don't want to breath MDF dust. The wood dust is OK, but the chemical used to glue it all together and treat the wood are mildly toxic. A simply paper dust mask is like sufficient.
Again, other than working out the minor details, you should get very good results from this project.
Steve/bluewizard