I use velcro tape. If you dry the area on both sides of the switch, you can get the tape with the hooks or loops to stick. Then, every week before Shabbos it's just a matter of attaching the other piece across.
There are items manufactured specifically for this which are sold in most judaica stores.
I would suggest only using 3M brand, not the junk you can ...
If your freezer is anything like mine it doesn't have an actual bulb.
One way round this is to use a powerful magnet which cuts the circuit without harming the freezer in anyway.
The Federation Beis Din in London have a dedicated technology site to deal with various home appliances and the halachic pitfalls that sometime follow on Shabbos.
Try taking a look ...
It depends on the freezer, but one approach is to just unscrew the light bulb if you can reach it. That's what my family has always done.
You can leave it loosely hanging in the socket if you want to put it back in for during the week, though unless it's in a dark basement it's probably easier to just leave it out all the time.
This only works with basic ...