Another inexpensive way is to line your closet, (actually doing just the back wall has worked for me,) with cedar flake board sold at many of the home center / lumber yards. It's 1/4" thick, can be put up with nails or screws, and if it ever starts to lose it's aroma, just scuff it up with a piece of sandpaper and the aroma comes right back.
Respectfully,
David