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.
Baxton Studio Aisopos Modern and Contemporary Grey Fabric Tufted Twin Size Daybed with Roll-out Trundle Guest Bed
Home Sweet Home
queen bed with trundle - Google Search
The Land of Nod | Kids Trundles: Light Blue Storage Trundle in Beds
Ultimate trundle bed. More
Create a cozy sleeping nook for company. Many of our daybeds can turn from a single to a double, and you have a choice of mattress to get the comfort you prefer
Red, White and Blue
AMAZING! DIY trundle bed. 2-4-1 bed situation for our small spare room!
Syd's Room Ideas
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
Simple twin bed trundle plans. This trundle is so easy to make, only takes an hour to build.
Project Plans - free
Triple Trundle Bed! Great for kids bedrooms & sleepovers! More