Ptchap - your best bet is to go with tension rods or bungee from side to side. Many nice ideas posted that work well in a convenientional trailer or tent trailer but won't work at all in an A-Frame trailer.
A good thing to keep in mind is that altho you posted on the A-Frame sub-forum, your post appears to the entire FR forums and many do not realize they are responding to an A-Frame question so you just have to sort thru the answers to see if it would apply at all.
2012 FR Flagstaff T12SDTH
1996 Shadowcruiser Pop Up Truck Camper
1967 Newell Motorcoach
2003 Ford F150 5.4 V8 Triton Super Cab
2004 Nissan Titan LE 5.6 V8 4x4 Crew Cab