I've not got any cages in mind off my head, and being so personal an item it's hard to be specific about one so I'll give general advice!
If you have a foreskin, I'd highly recommend a cage with openings, ideally lengthways bars are best, so you can retract whilst wearing it. This makes living with the cage and prolonged sessions much easier and cleaner. (But don't forget to arrange monitored cleaning openings!)
Metal are often more uncomfortable due to their weight, plastic being lighter can be better, but they're often totally inclosing or very tight bars to offset their weaker materials. Careful of cheaper cages as they can have pinch points and exposed or rough surfaces that require sanding down. Always test wear a cage before committing to a longer session!
You'll probably want a supporting belt for it. (This stops erections pushing the cage outwards away from your body) But you'll have to accept that no cage short of a PA piercing or full "belts" are inescapable. It's entirely possible on a cold day, to pull the cage off and release your balls from the ring too! Don't chase having a smaller and over tight ring to try and prevent this happening. Some cages have a ball restraint bar, allowing an open ring, but trapped balls. It's very rare, I've not personally tried one. Chastity is part honour and part wanting to submit with the physical cage being as much a gesture of that control given as a barrier.
I'd start with something that fits your flaccid size and comes with a range of rings to find the right size, you don't want to have one so tight it cuts off circulation! So be careful. Go in fully expecting your first few cages will be educational, and ultimately replaced!
Full belts I've not really tried much, but can be quite tricky to get right, you'd want one with a full front covering plate to prevent pull out. Full chastity belts in general though are not a good idea for a starter!
If it's just for play sessions of a couple hours or an afternoon/evening you've got a lot more freedom and flat ones for example can be quite fun if somewhat impractical for longer wear generally.
If you're handing over the keys, some numbered plastic tag locks and a lock box for a personal backup key is recommended, so they can verify if you open it (by breaking the tag lock) and you'll have a safe way to get out if there's an emergency without the need for them to rescue you.
Ultimately you'll have to experiment to find what works for you. I've never personally found that "perfect" cage.
You're welcome to ask me any further questions though!