Okay, not to monopolize, here, but I DID step away for a half hour.... *heavy sigh*
If you were to ask 5 people on this forum as to what substrate to use, you will 5 differing combinations. Personally, for CP's, I mix sand and peat (pool filter sand and the Canadian peat from Home Depot) and then topdress it with LFS (Long Fibred Sphagnum - also from H.D.) Sand provides drainage, analogous to gravel. Be sure to throughly rinse whatever subtrate you use.
Hmmm... what to do about sloping woes? Perhaps you can place a plastic border along the perimeter of the sloped part to hold things in? I think a moist substrate would naturally keep itself from giving way, somewhat.