In my experience running tight O11 builds over the years, I saved some cash using the EZDIY-FAB 12VHPWR 90 Degree Extension Cable. It clears the glass perfectly and only costs about twenty bucks.

TL;DR: Skip CableMod and get the official Corsair direct cable. Its cheaper, safer, and plugs directly into your RM850x. I went through this exact issue with my O11 build. Honestly, the safest and cheapest route is to just buy the official Corsair Premium 600W PCIe 5.0 12VHPWR Type 4 PSU Cable. It runs about twenty five bucks, so well within your fifty dollar budget. Since it plugs directly from the PSU to the GPU, you dont have to worry about bulky adapters or melting risks from cheap third-party extensions. The routing is way cleaner and the bend is much tighter than the stock Nvidia adapter. CableMod makes decent stuff but their angled adapters had major issues. Their direct replacement cables are fine, but the Corsair official one is cheaper and guaranteed to work with your RM850x.

Honestly, if you're worried about the glass clearance and melt risks, you might want to consider just getting a new power supply that natively supports the standard. I know that sounds like overkill, but I really dont trust those bulky adapters or even some of the third-party cables when it comes to high-end cards like the 4080 Super. IIRC, a lot of the issues stem from the connector not being seated perfectly flat, so no matter what cable you grab, just make sure to double-check that click. I think the official cables are fine, but I personally get super nervous about any extra connection points in the chain. Just take your time and be careful during installation. If you find one you like, definitely verify it's rated for your specific unit's pinout because even if they look similar, you could fry something if the wiring is pinned differently.

I was super paranoid about my 4080 melting too, honestly. If you want to avoid CableMod, a decent option is the Fasgear PCI-e 5.0 12VHPWR 16Pin to 3x8Pin Cable. It runs directly from the Corsair RM850x to the GPU, so there arent extra adapters to worry about. They use 16AWG wire which is standard for safety, and the cable is surprisingly flexible for tight bends in cases like the O11. It is only about twenty bucks, so it easily fits your budget. Just make sure you plug it in all the way until it clicks. Let me know if you want me to help look up the exact pinout specs just to be safe.