Any of those will work, but you want a rim to prevent any drippings from making a mess in your oven. In general, the lower the sides, the better for overall air circulation around the roast, but it is not essential. If you want to be able to deglaze the pan, avoid the ceramic or glass pan, as they cannot be used stove-top.
Using celery (and carrots) as a make shift roasting stand is very effective. They will add their aroma to the pan juices, which is often a good thing. (Note that by the time the roast is done, they are usually spent and not worth eating.)
IF you don't want the flavor from vegetables, any small rack which will fit will work. If you don't have one, you can improvize one by crumpling up aluminum foil into a long snake shape, and coiling or zig-zagging it on the pan just large enough to fit the roast.
For a slow roasted pork shoulder, you could also go without any rack or elevation, at the cost to some small amount of crust development on the bottom. Depending on your desires, this may or may not matter--when I do an indoor pork shoulder for pulling, I don't bother with a rack.