I purchased custom bench cushions for outdoor seating at cushionsource.com. I thought they did a very good job. You're cushions might be considerably more expensive, because I'm assuming you're going to need a thicker mattress. My 4" thick, 8 foot long bench cushion ran about $225.

The problem with coils/innersprings is that if you sit on them a lot they break down on the front edge. And they dig into the back of your legs as well.

I have a "sleeping bench" on my summer porch, that needs a 32x79 mattress. A Semi truck sleeper mattress fits perfectly. It comes rolled in a box, delivered to the door for about $300. It's about 5 years old now, and is used very often as both a bench and a bed. (I sleep on the porch in the summer) It's held up very well. The only annoyance is that memory foam is temperature sensitive, and it's very stiff when the temperature drops below 40F.

I purchased mine at mattressinsider.com They do custom sizes as well.

I purchased custom foam seat cushions from CushionsExpress and am planning on ordering foam for a headboard from them as well. If you go to the link for custom cushions : http://www.cushionsxpress.com/catalog/Custom_Cushions-2-1.html you can enter in your dimensions to figure out price.

The Indian or Thai mattresses (very thin, but very comfortable) are made to sit on top of a flat wooden bed/daybed and are very, very comfortable. I've slept on one for months at a time while in India. You can buy one, or order one, from many Indian furniture stores. Just go to your local Little India and ask around.

I ordered foam cushions from eFoamStore.com for a giant window seat I built. I went for the higher quality foam and it's still holding up great a decade later. I opted for 4" thick foam and it also works well as a guest bed. I hope that helps.

I did something similar about 10 years ago. I was able to purchase a custom sized futon mattress from a high-quality place and it cost me about $175 (but that was 10yrs ago). It was more or less twin sized, so when I got a queen bed, I still kept the twin futon around for stay over guests. That futon mattress is unlike any one around. It was the best sleep I've ever had.

Urban Outfitters sometimes carries Indian style mattresses too. Also, probably a furton mattress might work.

I don't know if this will work, but worth a try.

Ikea has foam mattresses. Buy one size too big. Take it out of it's cover. Cut it down to size. Put back in cover(I'm guessing it won't have too much excess), close and fold in cover. Use flat sheets instead of fitted for mattress. We always use flat sheets, but that's another different tip. We have a full bed and used a queen flat sheet instead of fitted. It fits thick mattress better, and folds better in the linens.

Another thought is to get a twin and if it's too small fill in the edges with some found thin pillows. you wil only sleep on the twin, and the pillows will only be visible filler. Use my above flat sheet suggestion.

I got a custom sized mattress for my vintage sleeper sofa through my upholsterer.

It's not a high end mattress for every day sleeping, but guests have found it to be pretty comfortable.

You might also consider a custom high-end futon.

I had this same issue with an MCM daybed. I went with firm foam with a separate topper.

You could go with foam but thats not going to fit as well with the style of the bed. I would go with the suggestions of Indian/thai mattresses as well. The next best thing would be the custom futon mattress. Please avoid the Urban Outfitter version as it doesnt hold up well over time.

Many futon places make their futons from scratch. Especially the non-chain places. It would be simple enough to call one and inquire about a custom sized order. This one isn't too far from NYC... http://www.whitelotus.net

What about a twin-sized japanese futon?

The ones from JLife aren't terribly expensive, they are supposedly comfortable once you adjust to a firm mattress, and they roll up easily so that you can "reclaim" the daybed as a sofa during the day if you would like to.

Gardner Mattress in the Boston area ships all over. They do custom mattresses. I think they are terrific. www.gardnermattress.com.

Call Dixie Foam in Manhattan and see what they can do. They make mattresses, so they should have foam more appropriate for sleeping. I have been sleeping on of their mattresses for years.

Foam mattresses, even memory foam, can be cut to size (I used a bread knife. Seriously). Have a custom slipcover made, or make it yourself.

World market is currently selling a daybed similar to this-maybe they sell the cushion seperately?

Talk to my Upholsterer in Queens he may be able to help you:

Galeano's Upholstery in Sunnyside (Queens) - 718-396-9816

Honestly. they sell thick foam - up to 5" - at JoAnn Fabrics (and other fabric stores of course) and it might be feasible and far less pricey (use a coupon!) to attempt making your own!

An alternate tip: if you're consider cutting down a foam mattress to size: electric turkey knives cut perfectly through foam. Seriously, you won't be disappointed.