MDF shelves cost £££££££££££ if you buy one ready-made! Much better to make your own: buy some MDF, use your standard shelf as a template to mark the shape, cut it out with a jigsaw, make some holes for speakers and cover it with acoustic cloth. Sounds much better than standard crap and won't bend/break with the weight of the speakers.
Also, mount your speakers on the underside of the shelf so that no theiving scum can see them under the cloth (but you can still hear them). I didn't do this and some complete b*****d nicked my homemade shelf complete with JBL 6x9s!
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Fart In A Tin