Don't bother building them, they're just off-the-shelf 14-pin resistor networks. Radio Shack doesn't seem to have them but Mouser Electronics has them for 56 cents each:
652-4114r-1lf
Figure out your frequency for each crossover point, then divide it into 7200 to find the right value in KOhms.
For example, say you want your midbass to midrange crossover point to be 300hz:
7200 divided by 300 is 24, so find the closest one you can to 24KOhms. It happens to be Mouser part number 652-4114R-1LF-24K.
Then say you want your midrange to tweeter crossover point at 3200hz:
7200 divided by 3200 is 2.25. Well they don't have a 2.25KOhms resistor network, but part number for 2.2KOhms is 652-4114R-1LF-2.2K and part number for 2.4KOhms is 652-4114R-1LF-2.4K.
So, you just do the math in reverse (7200 divided by KOhm rating) and decide if you'd rather try the 2.2KOms one for a crossover point of 3272.7hz, or the 2.4KOhms one for a crossover point of 3000hz. The 2.2K one is probably close enough.
It's a crossover that looks more complicated to set up than it is, just look at it for a while and it'll probably start to make sense..
Hopefully this helps.