I stitched my own stack pack. Since I made it, I could control just how high it sat on the boom. Plus since we have a loose footed mainsail, it was easy to make and so easy to use.
The height of the cover is determined by how high the sail cars stack up in our strong track. The stack pack for us is on sail slugs in the boom with the top edge attaching to our lazy jacks. My one suggestion is that if you try to make one (which is not very hard), do make the top gusset where the zipper goes narrow. If it is wide, it tends to channel water into the sail and you will find a shower awaiting you each time your raise sail. I had that issue in my first fitted cover but with taking it in so the channel that held the zipper was only about three inches, we no longer get a wet sail.
I have a block at each end of the cover with a long line attached to the zipper pull. I can stand at the mast and zip it open and closed with ease. I did not want to drill into my mast so I have web loops that have a bungee cable that holds the cover forward to the fittings that are on the mast. Being bungee ties, they are forgiving at any point of sail. My cover is certainly not perfect but is totally functional and easy to use.
Hope you have found a solution. You are correct that with the length of the boom on the CR, if your canvas covers the cockpit, it is a challenge to get to the end to attach the traditional removable canvas covers.
Best regards,
Leslie
s/v Tango, CR 34
Lankford Bay Marina
Chester River, MD