I'm really glad you like them! It's made my job easier as I have all my polishes in a safe place, and clients love it when I whip out my board... they are like kids in a candy store, chosing from 40 polishes at a time.
I like black because it shows all the colours off well, but if you are making your own, the sky's the limit! I thought a black denim one would look cool, and I've done pink fleece and black satin. I got supplies from B&Q (MDF, hinges, screws) and Ebay (1" black webbing, polar fleece, elastic... I would have bought these at a fabric shop but at the time, our local one had just closed)
I take an MDF board, and drill rows of holes into it, two holes for each polish. I cover the front of the board with fleece (I like fleece as it's padded and keeps the bottles cushioned a bit - if you want to use a different fabric like cotton or satin, I would put a bit of padding or a layer of fleece under it) and tack it down so the front side is smooth (leave the back of the board exposed). Then take a large upholstry needle and thread elastic through the holes and over the fabric on the front, making a "seatbelt" for each bottle. * Remember to leave some slack so you can actually get the bottles in!! Because the back of the board is exposed, you can see the holes and where you are putting the elastic. Fasten the ends of the elastic to the board, then firmly pull the fleece over the front and back surfaces and staple (like you would if you were recovering a chair pad).
Make two of these. Attach a handle made out of webbing onto one of the boards (screw it into the MDF to ensure it doesn't come loose!) - you can do this before you finish stapling the fabric down so it looks neat. Use hinges to connect the two boards, and add a strap to keep them from falling open (make a small cut in the fleece, push a length of webbing in and staple to fasten; repeat on the other side).
For the elastic, I've used both thick elastic (1/4") and thin, and I have come to prefer a double thickness of the thin. This gives you enough give and stretch to secure the bottles, but fits better into the lids. I open each lid slightly, then slip the elastic in and tighten the lid. I have never broken a bottle of polish when it's been on my board - they stay where they are put.
I like crafts and DIY, and just made what would work for me, but I'm happy to help others make their own. PM me if you need a hand - I've made lots of these and you might as well learn from my mistakes!
BTW, I want to see pictures when you're finished!