Unfortunately the challenges are suspended for 1 year, so you cannot make any new challenge now. However, the rules of challenges not allows to make challenge based on list of allowed caches. But you can made challenge based on at least 90 caches in certain area (defined by polygon). However when someone else puts other cache in the same area, his cache will be valid for the bonus.
Hopelessly, as I've wrote above, now the challenges are suspended, so you can invent some other sollution
Maybe something like this:
Every cache will have a number written on it's logbook. In the listing put table 10x10 of empty cells with all row and collumn checksums. The cacher should fill this table with the numbers found in the caches. Missing numbers can be calculated from checksums. This system allows fill the table knowing at least 81 values, so founding 81 caches. More caches will be found, few equations should be made to fill it.
Now found some simple equation fo calc the final coordinates from this table, possibly using all values (maybe calculating discriminant of the matrix). But think about the geocacher in the forrest without the internet connection with wet paper with 100 numbers written by hand in small cells of the table with smartphone instead of spreadsheet - will be hard.
