This underbed storage is for an ensemble bed. "Storage" for a guest mattress is for anyone with a double/queen/king mattress.
Basically we wanted to buy a Brimnes bed from Ikea, and after a year of ummming and arrring about this we went through the trauma of going to Ikea, but all the Queen Brimnes bed bases were out of stock. We made our own version. We did buy some cool fabric from Ikea to help with the guest mattress storage.
Step 1: Attach legs
Ensemble beds usually have screw castor wheels. Have a look under your bed to see if you actually have these types of wheels. If so then buy the appropriate number of wheels for your base. These are available from large hardware stores (in Australia Bunnings). We selected the 23 cm (10in) legs. Alternatively these guys do cool long bedlegs with recycled material (but it does require more effort than what I did): http://www.universalbedlegs.com/
Chuck your ensemble base on its side. Uncrew old bed legs. Screw in new bed legs. Carefully put flip yourbed horizontally again (this is a two person job).