If you have a tramp that does not have lacing down the middle, I think the best (and easiest) idea is a fairly deep flap all the way across the front of the tramp (open edge facing aft), with the front and side edges sewn to the tramp. As far as whether you want the thing to be totally open all the way across the back or divided into compartments or use velcro or grommets or whatever, that is an individual, creative thing. If you have a mesh tramp but want the stuff in the storage area to be able to be kept relatively dry, you can make the whole flap section, bottom and top, out of a waterproof material.

I like a similar flap, just mesh, not as deep fore and aft, all the way across the tramp at the aft end, but with the open part facing forward. That would be for storage of items that do not need to be kept dry and are not too bulky. And it would be nice to have that aft flap closed with velcro or something so things cannot get washed out of it or washed into it and so you can't trip on it or whatever.

These are just my opinions as a crew who likes a clean deck and the ability to stuff something quickly out of the way, but be able to have it easily accessible.

(If you have lacing down the middle of the tramp, you would have to divide the above into two compartments on each side both on the front and back of the tramp.)

If you want a spinnaker-storage "compartment" sewn onto the tramp, that would be separate from the generic storage compartments, and design and location would be a matter of personal preference.

I HATE tramps with little storage flaps sewn on with the open side facing forward. They are so tight you can barely get anything into them, and when you try to get something out of them, you risk losing it overboard over the front beam.