If necessary, you can even make trailers at low-cost or give them away to certain agencies or individuals if you think the visibility this brings will help your business. Bicycle trailers are easy to design. They must be as low as possible so they do not tip over. They should be fairly narrow to negotiate traffic safely, but not so narrow that they are tippy. Because weight is a consideration, you'll want to design them with no suspension or lightweight suspension.
Suspension is a good idea, because unloaded trailers do get very tippy when hitting bumps.
The trailers should hook to a spot as low as possible on the bicycle, but not behind the bike. If the trailer connection is behind the bike, the bike's performance in a quick stop is not desirable. The usual points of attachment are the left chainstay near the dropout, or under the seat. The actual connection has to flex in all directions, since the bike will lean around a corner, while the trailer remains on-plane. A common connection technique is to tie a connector mounted the bicycle to the trailer with a leather strap or rope.