By the information provided, I can only assume that there are no amp repair shops in your area and you don't know anybody with woodworking tools. Although you could go to a lumber yard or supplier and give them the dimensions, I recommend approaching a cabinet maker in your area for the job. Take the entire cabinet to them, or to make it easier, if the bottom panel is the same size, take that to the cabinet builder so that it will serve as a reference and they can simply make a duplicate of it. To an experienced wood worker, this is a very simple job that can be done quicker than having lunch. If you want it covered with sheet vinyl or Tolex and they won't do it, they may be able to provide you with a reference, otherwise, inquire at an upholstery shop. Or, if you're up to it, you can purchase the cover material from an online supplier and apply it, on your own. When I sealed off the top on two of my Boogie 4x12s, I used what I believe is 3/4" plywood sheet, although it actually measures at 23/32". I just used regular ol' plywood, raw and unfinished- kind of ugly, but it works. I cut it to size on my table saw. When I drilled the mounting holes, I can't recall if I measured them out, or if I used the bottom panel as a jig. It was quite a while ago.
