Ahh, good you have a daily so you can work on the Supra, that is the best way to do anything I think. I have my Rangers so if the Supra goes down I have back ups.
What I would do for that, if you plan to paint the mating surfaces, is put painters tape around the inside wall leaving a inch or two beyond the opening. Then put some paper in there, just don't push it to far, lol. It should look like wads of paper surrounded by tape. If you do that you should be fine.
If you do not plan on painting the mating surfaces then just put painters tape strait along the flat mating surfaces.
Make sure when you clean the parts with the brake cleaner that your not on grass, cus it will kill the grass quick, lol I learned that the hard way! It also has a heluva punch to it when you first get a whif.
By the way, the intake plenum on our cars, is what I think is the hardest to paint neatly. I would suggest some real fine sandpaper and primer for it, I have seen a few people try to paint them with spray cans and the paint looked horrible, because they didn't take the time to prep it.
As for the sandpaper, I would go from a medium to high grit number, I would start at maybe 800 or 1200, if 800 seems to coarse then go with 1200, lightly sand it all, then move onto a higher grit number, the higher the number the finer the finish. I would only use 800 if there is a lot of dirt and debris that the brake cleaner and wire brush wont get off, if there isn't much there then start with 1200.
Now I am not absolutely sure about what primer to get but since the plenum is aluminum, I think any kind of metal primer should do fine. After you get the cleaning and sanding done, then start the primer, paint the primer down just as if it was your final coats, take your time and it will turn out great!!
Good luck!!
Here are two pics of the tape ideas:
