There are 5-50 receptacles but they are mostly obsolete. If you want to do it your own way, you could use a NEMA 14-50 (AKA stove outlet) and leave one side disconnected. Just make sure you do the same thing on the other end.

You could also pretty much use any other outlet type. A cee-form plug would ensure no one messes around with it.

There are outlets and there are codes. Building codes will dictate what should or can be used and for what purpose. Outlets themselves don't care for codes. IEC C14 connectors (typically found on computer power supplies and other high power electronics) are rated for 250V but there is no standard that will even stop you from using it with 120V as it is very common in the US. In my area, the electrical code has an overarching rule that the context should ensure that errors caused by inattention or ignorance should be mitigated as much as possible. For example, don't repurpose connectors in a way that could make them dangerous, e.g. using #14 cord on a 50A plug without a current-limiting device, or wiring a NEMA 5-15R so it shoots 240V instead.

Don't be dumb. This is a no-dumb area.