Many folks have a collection of toys that they wish to have handy, yet secure from prying eyes. How about something of a locking hope chest or an under-bed chest? Once you are good at welding perhaps a bed frame with locking drawers and or some short of hidden compartment. The buyer can store some bed linens so camouflage the goodies.
Saint Andrew's crosses are popular. But many, having little space at home, and a need to keep them out of sight when not in use, require them to be stowable. Could you design one or two that can be easily assembled, disassembled and stored neatly under a bed or in the back of a closet?
Most dungeon furniture have the problem that they have a low demand and or tge need to be quite tailored to a specific size, need, style, even decor. Yet building a bunch of prototypes that may never sell would be prohibitive. So having a bunch of drawings of what you could build might be an answer. If you are an unskilled with pen and ink, perhaps you can have a art student try their hand at converting your sketches.
The types of dungeon furniture include and not limited to:
* spanking benches (many styles here)
* racks (for body holding rather than stretching)
* canopy beds
* thrones (very many styles here)
* horses (simular to vaulting horses)
* adult sized rocking horses
* adult sized highchairs
* adult sized baby cribs
* stocks and pillories
* queening chairs
* human toilet boxes / housings
* coffin beds
* coffins with ventilation (and maybe O2 / CO2 detection) such that they may be closed.
See the furniture of films made by Insex, Infernal Restraints, Kink dot com, and the like. Including a link to clips from these films may bring extra live to your advertising. There were many films of the '40s '50s and '60s that included dungeon scenes. Some of them set in the future.
Have a "Give Me Your Idea" page on your site where folks can send drawings and descriptions of what they would like.
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How about a standard sized door made to look like The Tardis doors?