What Do You Use For Dehumidifying Enclosures/Domes?

We just use cheap desiccant pouches like these.  Not sure if these are the exact ones, but they are similar.  Normally just tape them in, but you can fold them to jam them into tight areas.  We replace them during the yearly cleaning or really whenever we get into the dome.

Hi Ethan, I notice that this post already one year old, but the humidity problem is always nipping at our yard...that's my case any given time as we have very high relative humidity all year long....so I think it may be useful to others who have similar problems.

Space is always tight and silica gel which was suggested in the metal casing or small paper/cloth bag is not gonna work especially when the silica is saturated with the moisture it has been absorbing and to make things worse is that these silica gel will get the moisture back out in these closed space and will add to the humidity in the air. 

We have tried a better result is using another type of moisture absorbing material made of lime desiccant, this process the humidity into something irreversible and the content becomes somewhat harden but it won't spit those moisture back into the closed space.  I can't buy that easily, so I usually asked the manufacturer to send me some along with the products.

Alternatively there is a membrane, made by 3M, I just don't know how to get them and it has to be integrated by the manufacturer, previous I dealt with design and they incorporate several small holes cluster design into the closed space to facilitate this membrane breathe as it allows the water molecule out but keep the moisture out.

I hope the above helps.