Water Filter Adapters?

I have a two water filter housing that hold 20" long 5 micron and 1 micron filters.  I also have a large collection of 1 micron carbon activated filters that are 10" long that someone gave me, still new in the original packaging.  There is an adapter out there (somewhere) that lets you put two 10" long filters end to end to replace a 20" long filter.  But I can't find it.  I would rather buy, but it worst comes to worst...I can machine it out of aluminum.  (I could machine a new car out of aluminum, too, but it might be an inefficient way of doing things.) Suggestions?