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).
Step 2: Use Your Storage
You can store anything under your bed now. We got already had these handy containers on wheels. They now fit under the bed!.
The one advantage of not being restricted by proper drawers (like Ikea's Brimnes bed), is that you can choose to store long things...like a wedding dress.
Step 3: Create Cool Bed Head & Hide a Guest Mattress
We have a queen bed. Our guest mattress is a double. While we've purchased a sofa bed, I just hate the mattresses on those things so I wanted to keep the good double mattress for our guests. But where to store it?? Well we've used it as a bed head for our new raised bed. We've just thrown a piece of fabric over it. (I think it's important to pick a fabric that makes you happy - whether it's your favourite colour or a cool print). You just need the fabric to cover the sides that are exposed. Essentially you wrap the mattress like you would a present. Then you use pins to "tape" it up.
The raised bed makes me feel a little bit like royalty. The soft bed head is great to sit up against, and makes me feel like spending more time reading/surfing the net in bed.