You are correct, wood duck flank feathers are tough to tie in whole... A lot of variations I have seen use the slips you mentioned, but some find them harder to tie than whole feathers. But here are a few things that should help...

Choose smaller flatter feathers from the middle of the flank rather than the huge ones. Look for the smallest matched pair as they will still be plenty large for a size 6 salmon fly hook. Leave the stems as long as possible until they are tied in and the head wraps are being placed.

Before tying them in place the prepared flank feathers between wet paper towels and microwave for a few seconds. Just enough to get some steam rising and leave them there until you are ready to tie them in. You can steam them if you prefer and then place them between damp papertowels if preferred.

Finish tying off the collar with the stem on the near side pointed up and cut off flush with the top of the hook shank. That will create a smallish flat right at the start of the head.

Make a tiny thread dam at the very back of the head as you are getting ready to set the wings. I tie almost exclusively with extremely fine thread and usually make just 6 wraps (two side-by-side then one through the middle, repeat atop the first three) but thicker thread should take just three wraps.

Place the warm wood duck feathers back to back and in position. Note how the stems ride as they cross over the thread dam and then crease the stems with a fingernail so they are flattened with the flats riding over the dam flat or slightly high to the middle.

I prefer to wrap both in at once and usually that is fairly easy. Wrap around the stems right on top of the thread dam but pinch the thread between your fingertips on the way up and then continue down around the stems just as close to the first wrap as possible. Then tighten the wraps by pulling STRAIGHT up gently.

If you continue to have issues tie in a very small bunch of unbarred wood duck fibers right in the middle of the top of the head to separate the feathers slightly and prop them up. Keep the bunch as round and tight as possible and tie it just in front of the dam.

Do not trim the stems until the head is mostly wrapped. If tying for presentation I will often put head cement on before finishing the wrapping and let it dry so I cannot knock the feathers loose.

Good luck and I hope this helps.