The Luggable Loo toilet seat with lid fits onto a plastic bucket. It is designed for use in emergencies, when camping, or whenever indoor plumbing is not available. There are several brands of toilet seats of these types on the market; all of ours have come from Emergency Essentials and are similar to the one they are currently selling under the name "Tote-able Toilet."
The toilet seat is fine with short term use, but when used daily by three people, the plastic lid will crack within a month. Here is one such lid after about 3 months use.
This is the third seat that we've used and every one of them has cracked soon after using it. (Note: The lid was removed purposefully because of the tight space. It is easier to just set it on and off and not have it right against your back.)
These lids just don't hold up when used over time. A much better solution would be to build a plywood box that would hold the bucket under regular toilet seat.