I am guessing the fabric weave is too wide, you need to find a fabric with about 300 ct. weave then make a cover for the pillow, then you can use your slipcovers. My comforter fabric is very fine, so no feathers come out, maybe a flat sheet with a very high thread count will work, see what you can find, 300 or above. hope this helps Rene

You could make an inner lining with pillow ticking, it usually comes in stripes or floral. You can buy that at JoAnns, not very expensive and the feathers shouldn't escape. If no JoAnns nearby try buying a sheet or pillow case with a very high thread count and use one of them to make your liner. I can't remember what the thread count is on ticking.
Kathy G in MI

Or buy a set of 800-1000 count pillow cases and cut them to size for your cushions. Figure out which option is cheaper.

Thanks for the ideas- I'll try making an inner lining using a thicker weave. You would think that the manufacturer would have used a thicker fabric for the inserts. I guess that's why they were only $12 each. Go figure!