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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.

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from hayneedle.com

American Woodcrafters Heartland Trundle Bed, Size: Twin - AWR759-1

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from IKEA

Hemnes

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

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10

Sofa by day and bed at night, most of our comfy day beds even have built-in drawers to store duvets and pillows. Many can turn into both single and double beds and you have a choice of mattress to get the comfort you prefer.

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from Babble

8 Incredible Built-In Beds for Your Kid's Room

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