Hi, welcome to the forum.
I am posting a link to an auction just to give you an idea of what I think is a nice holder.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26378&item=5194297847&rd=1&ssPageName=WDVWThey make this in a size called PC 5 II, PC 6 II and PC 18 II.
The higher the number the thicker the card it holds. No one is making a holder for the Really thick cards like the "Monster Book of Monsters" yet, anyway that I know about.
They refer to thickness of the cards as " pt" or point. This is in mils (1/1000 of an inch). A normal card is about 50 pt, the "Monster Book of Monsters" is about 187.5 pt (3/16 inch).
A lot of techno jargon, but basically the thicker the cards the higher the "PT" number is. I think that 180 pt is the thickest currently being made Screwdown holder.
A couple of more interesting links:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=26378&item=5195311795&rd=1&ssPageName=WDVWhttp://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=55857&item=5196032386&rd=1I recommend getting a small sample to see what you like. It'll save you from having to swap cards later to the one you ultimately end up liking. Remember NO PVC! (Poly Vinyl Chloride - it wrecks the cards)
Good luck on your boxes.