I run an open trailer. I have air hose snap fittings that hook up to both PW reel hoses. I unhook at the pumps and burners then blow 100psi for maybe 5 minutes though each. The same with the supply hose blow back into the float tank. Then I dump 2-3 gallons of RV antifreeze in the float tank. Start each PW until purple antifreeze comes out and then kill them. You have to pump a bunch of air or it can leave water trapped in the reels. It's drag. I do this all winter if it's dropping below 25 degrees at night. If it's staying that cold during the day we are out of business for a while. I have a heated shop but too much stuff in it. They sell the RV antifreeze at Lowe's, Auto Zone pretty cheap.
An enclosed trailer won't solve the problem unless you have a heat source. I've really thought hard about an enclosed trailer and then just put a 110v strip heater in and plug it in every night. But I really like being able access everything from the sides. I also dread having to relocate all the gear.