Everything depends on how large a terrarium you are talking about and how large will the orchids get including the height of the flower spikes.
Growing a plant in a terrarium has its own challenges that have to be addressed.
I would think that you need to identify those issues in the home that prevent you from growing an orchid inside your home.
Check out the link for an overview of orchid culture and perhaps try and see if you can adjust your culture in the home to make life tolerable for your plants.

Here is a link that might be useful: Orchid culture

Welcome. What part of the country do you live in? Different parts of the country require slightly different culture.

1 - Thanks for the link - a great deal of excellent information. I really don't know how big these orchids might get or even what type of terrarium would be used. I'm very much in the planning stage right now.

Here is the reason the issue of a terrarium has come up at all. Previous orchids brought into our home have died. Numerous people have told me that orchids thrive on 'benign neglect' - but, either my 'neglect' has not been 'benign' enough - or - the other complication is destruction by house cats.

As originally stated, these 3 orchids are still in the original pots with the original plastic around the plants. I left them like this because I realized that, with the plastic around them, the cats left them alone.

Initially I considered putting them - individually - into very large and tall glass vases. That is, the entire plant, pot and all would go into the vase, which would be a type of terrarium. From other feedback, this does not seem to be the best option. (I got the idea from the orchid display at my local Trader Joe's. Unfortunately, I found out that their 'system' is to water a blooming orchid well, put it in the vase and leave it alone until the blooms and or the plant died. At that point they toss that plant and start with another plant.

2 - I live in southern California - inland. Also, we usually do not heat our home as much as others. It may get up to 70 degrees in the winter, but is more likely to stay at about 65 - 68 during the day. At night it goes down to 60 or maybe a bit lower.

In the summer we don't use air conditioning, but do use a swamp cooler or evaporative cooler, meaning more humidity than homes with air conditioning, but still not a lot of humidity. In the summer the inside of the house is often 75, maybe even 78 or 80 during the afternoon. This would a slow climb from the morning. Typically we let the evaporative cooler run all night, getting the house down to about 60 degrees. If we keep eastern windows covered during the morning, the house stays cool until the afternoon.

- thanks, Cecilia

The most important consideration in orchid culture is light. Give the orchids as much light as you can without burning the leaves. Introduce them to more light slowly. With plastic wrapped around them they may cook. An orchid will survive in low humidity but they won't survive without enough light.

Can you put these plants outside? It sounds like both plants would like your conditions. Oncids like cool, and the Den needs high light, but can do fine in winter with 60 degrees.
A Den in a terrarium would be next to impossible unless you have a dwarf variety. Most likely a hard-cane phal type which get tall with long spikes.

You need to get the plastic off the plants, they will die. You need to learn about both plants to grow them. Both are fairly easy plants with different requirements regarding light and water.

Forget the cats, move the plants to a spot away from them or put something in front of them to keep the cats away. You cannot grow either plant wrapped in plastic. They need air!

Learn about what you have and put them outside if you can't manage them in the house.

Jane