Quick and dirty.

I had an unused heater vent with no cover measuring 90x300mm, and made this as a safety measure so people stop stepping into it.

The model was split into identical halves to facilitate printing on an Ender 3.

I had good results printing in PLA at 20% cubic infill with no supports.
At room temperature I was able to stand on it without any structural problems.

If you want it to be able to tolerate the heater being turned on, I'd recommend ABS or similar instead.

The best adhesive I've found for joining the two halves is silicone (Selleys All Clear).