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Remove the top twin-size mattress from the box spring on the daybed.
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Set the twin trundle up next to the daybed with a twin-size box spring on it.
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Lay a piece of particle board over the box springs. The particle board should be cut to fit over both box springs, so it needs to measure about 70 inches by 150 inches.
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Lay the king-size mattress over the particle board. Secure it all with a fitted sheet.
Check out this related video from Homesteady on Youtube.