Excellent item and was very easy to assemble. Putting the Day Bed and Trundle together took 30 minute - 1 hour with one person assembling. I can confirm that the trundle will only fit an 8" Twin Mattress. 

We placed a standard 8" Twin Mattress on top. There is a slight gap (before putting sheets + comforter) and one could possibly use a Twin XL if you wanted an extremely snug fit.