I have toured with a 25l backpack bungied to the rack-top. You need about 3 bungie chords, at least one going under the rack and over the bag.
Don't let anything drag into the spokes or wear against the tyre.
make sure your rear lamp is not blocked.

I sometimes use a rack extender for carrying big items such as bags of plaster, cement or horse manure!
I have a 1/4" plywood length with 4 pairs of small holes, threaded with a length of wire so i can fix it in place and remove it easily. If you want a more permanent fixture, corrugated plastic fixed with zip-ties should be strong enough.