Oh, one last thing to mention:
The thickness of the material affects unwanted resonances in the box. The thinner the material, the more the box can have undesired resonances, which would necessitate more inner bracing to get rid of them. That, in turn, would reduce your interior volume and the vicious cycle would continue – enlarge the box, a little more bracing, a bit more box size… until you balance and by then you’ve strayed away from the original dimensions more than you’d like.
When using the 18 mm instead of 3/4″, that’s only ~ 1/25″ less, which is actually well within tolerance of the 3/4, which can vary by quite a bit more. So yes, go ahead and use the 18 mm knowing that it is a very compatible size exchange.
Also, here’s a hint for others when building boxes: the smaller the panel, the more rigid it is for it’s thickness. And if it won’t be subjected to massive pressures, it won’t have the same resonance issues. For example, using a 4″ driver with 2 mm XMAX in a 0.25 CuFt (~7 Ltr) box would be ~9.5″x7.5″x6″ inside, so 3/8″ ply could be used quite sturdily, while using a 10″ driver with 12 mm XMAX in a 2.5 CuFt (~70.8 Ltr) box would be ~20.5″x16.3″x10″ inside and would need at least 3/4″ plywood/MDF with bracing. It’s all a matter of scale and pressures.
/tangent off