This is the best trundle bed frame I've seen. 1x2 and 1x4s are cheaper than a $50 sheet of 3/4" plywood and will hold up better than MDF with no framing to hold up the middle from sagging.
AMAZING! DIY trundle bed. 2-4-1 bed situation for our small spare room!
Jenny Lind trundle bed in Azure. Land of Nod. If I have a girl.
Sarah and Nat's cabin
triple trundle bed - Google Search
Ideas For Your Home
DIY Furniture Plans | Get the free woodworking plans to build this rolling trundle bed for only $38 in lumber!
BHG's Time-Savers for Busy Families
Twin Over Full Bunk Bed with Trundle Bed from 1800BunkBed
Baxton Studio Aisopos Modern and Contemporary Grey Fabric Tufted Twin Size Daybed with Roll-out Trundle Guest Bed More
Fine Looking Spacious Pop Trundle Pops
Triple Trundle Bed! Great for kids bedrooms & sleepovers! More More
Stuff to buy
Ultimate trundle bed. More
school bus conversion ideas