I've built concrete speakers – right into the building structure in one case. Though I admit I chickened out and used wooden (all right, plywood) baffle boards, rather than moulding around the correct size oil drum or cylindrical whatever. Something I noticed, after a year in place they were still producing dust; acoustically irrelevant, but poor WAF.
Wouldn't like to tour them, though, or use them as OB monitors or for live recording. Don't much like MDF for that matter (that is the trendy name for chipboard, isn't it?). Not just weight, they tend to fall apart under impact stress, even if you put chicken wire reinforcement into the concrete.
One trick I picked up from the Beeb for lightweight acceptably low resonance cabinets was to use relatively thin ply (19 – 23mm) (good quality, though. I'm afraid getting cheap ply isn't worth the savings) then bond fiberboard to the inner surface to damp it down. Mount the driver, and attach a sweep frequency generator. With your fingertips you can feel the points where panels start to flex at specific frequencies; mark these, and add bracing only where it will be effective. A lot lighter than the equivalent build homogeneously.