There's your problem... as soon as you covered the pit over you starved the briquettes of oxygen so they wouldn't have kept burning.
Builder merchant would do them.
If you go in and tell them what you're after and have they got any broken ones you ought to get a good deal on them
Engineering bricks are best because they're fired at a higher temperature than ordinary bricks to make them waterproof
the briquettes were still well hot when we dug them up i think some on the top would have helped and a but longer lol but you are right rocks or bricks would have helped loads