Contractors are already submitting bids for the first step of President Donald Trump's signature promise -- a border wall with Mexico.

This first round of bids is only to design and build wall prototypes in the San Diego border area. The deadline to get them to the Department of Homeland Security is April 4.

While construction of the prototypes could start within weeks, it will be months, if not years, before construction of the actual wall begins.

The prototypes will be smaller than the eventual wall will be -- only 10 feet tall and 10 feet long, enough to demonstrate what a full-size version would look like.

The department will decide how many prototypes to build once it sees the various submissions, said Jenny Burke, a spokeswoman for Homeland Security.

Related: Hispanic builder bidding to build Mexico wall - "Work is work"

There are some basic parameters for the wall -- while it should be 30 feet high, the department says it will accept bids for an 18-foot structure. It must run at least six feet underground to make tunneling more difficult. It must also be able to endure at least 30 minutes of attempts to bore through it with a "sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools."

But beyond that, the bidders have a fair amount of latitude in how they design it. In fact DHS is actually not even committed to a wall. It also accepting bids for a border fence. The difference: A wall is completely solid, while a fence is a barrier that you can see through.

Trump himself has continually corrected people who referred to his plans as a fence.

"It's not a fence. It's a wall," Trump said at a January press conference before he took office. "We're going to build a wall."

Related: Trump wants a wall. Border experts want a fence.

But sources have told CNN that career officials at the agencies most involved in the process are likely to recommend a fence rather than a wall. They believe it is important that border patrol agents be able to see through whatever structure is built.

Despite the president's strong views about the wall, Homeland Security seems to be open to ideas about what other kinds of structures could protect the border. In addition to prototype bids, DHS has opened up a separate process to collect more general suggestions from outsiders, including contractors, think tanks, universities and even members of the public.

"DHS recognizes that industry, other agencies, and other private entities may have interesting and useful ideas about how we could proceed," said a request for proposals, which are due March 31. "We would like to invite submissions of any such ideas so we can consider them as we develop a complete and comprehensive long-term strategy."