Kelana12... First of all, congratulations on building a relationship where your husband is accepting of you engaging with other partners! It's a gift that shouldn't be taken for granted.
As others before indicated, communication is key. You are looking to talk with him about it, you're already positioning yourself for success. Before you take the step to talk to him, take the time to learn about yourself and your needs versus wants in this scenario. It excites you, that's great! Not to mention hot, and I'm a little jealous. But how long do you want him to wear it? Permanently, short periods of time, as part of your play, when you are with a partner? Is the chastity cage the first step toward other activities like cross-dressing or sissification? Will your husband wearing a cage result in more play time with him, or less? What cages excite you? There are many sizes and materials, knowing what really excites you will help the conversation later.
Once you know more about your desires, you will be in a place to discuss it with your husband. If the cage will result in sexual benefits for him (more play time, a more aroused partner, teasing that will lead to more intense play later, etc.) then you can bring it up at the beginning of an intimate encounter or at the end during some after-play. How do you bring it up? Straightforward and honest is the best - "I want to bring a new toy into our sex life" or something like that. Tell him that it excites you (if at the beginning of play time, you can easily demonstrate how much it arouses you) and then use your intuition from there. If he is obviously not into talking about it, let it go for now. He may need more time to think about it or may not be comfortable talking about it in that particular situation.
When the conversation happens and he is open to talking about it, that is when you need to be honest with each other. If he isn't into it, find out what about it he doesn't like. Is he worried it's a step toward something he has a hard limit on? Does he have a negative experience you aren't aware of, or an experience that he wasn't honest about in the moment? Is he not into physical toys as part of being a submissive? Does he have hygiene concerns? Does he think it will lead to complete denial when he is only comfortable with partial denial? Actively listen to him and be honest in your desires, and it will have a positive effect on your relationship regardless of the outcome. Either after the conversation or as part of it, you can shop for a cage together. It will make it an emotionally bonding experience and will only serve to improve your relationship.
At the crux of it, he has to want to. It may take a few discussions and he may put some hard limits on it in order to be comfortable enough to wear one for you. Others have suggested "ordering him to wear it." Some suggested withholding sex or sexual contact until he agrees. I highly recommend against both. Ordering him to wear one only works if your relationship already has that level of domination and sub in it. If your relationship isn't at that point, that tactic is more likely to result in a negative reaction and impact on your relationship. Withholding sexual contact to obtain a desired result against somebody's will is ...... and there is case law in North America supporting this. It may accomplish your goal in the short term, but if he doesn't want to participate in chastity play or isn't ready to, it will build resentment and your relationship will suffer.
You are in a relationship with a submissive many can only dream about. Don't jeopardize it for an idea that excites you. There is no prescription of what a D/s or cuckold relationship has. You can incorporate any aspects that work for the two of you, and exclude anything that doesn't.
Good luck, and let us know how the conversation goes!