We bought a foam mattress for our large bed from here:
Home | Foam Rubber CityFoam Rubber City
Only $200 and came with a cover. However, the cover was not pre-shrunk and shrunk to 3/4 size the first time we washed it.
We brought Homelet to the place and he cut it to fit, notched for the rear support and matched the curves in the corner.
There is a mattress size called 3/4 and you may be able to find it someplace.
Custom Mattress | Radius Corner Mattress | Cut Corner Mattress - MattressInsider.Com
They advertise free shipping.
POINT OF INFORMATION: Be sure to have some water impervious cover on the top of your mattress or your body moisture will migrate through the mattress, condense when it reaches dew point and form mold.
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