Female led relationships discussion

Hi everyone, I’ve got a slight problem. My girlfriend and I are both making an effort to find a cage suitable for long-term wear (we’re looking at 1-3 months now). We have a dominant/submissive dynamic that is deeply rooted in BDSM/ Bondage, so the logical progression is naturally for me to be under lock and key. I’ve tested numerous devices; none are custom yet, but they all share a common flaw: when I get an erection, my testicles turn a deep blue/purple. This usually takes forever to go down, especially if I’m performing oral on her (which happens often), which is always a huge turn-off and means taking the device off right away. Now that I’m considering moving toward a custom cage, what features should I look for to lower the likelihood of this issue? A bigger gap maybe? I’m a big guy with a 3.5” flaccid length and 7.75” erect. Any advice is greatly appreciated! Thanks in advance.