Can you help? I have a rock'n roll mattress.

I bought an IKEA twin bed, Hemnes is the name. It has some curved wood slats that fit on it to hold the box spring under the mattress. The curve is an upward curve. So I can't find a box spring to fit over it because most box springs have boards across them that sit on top on the curved slats so it rocks back and forth. I have to sleep right in the center of the bed without moving much at night because if I move to either side, it dumps me down the mattress like a playground slide (except at my age, with arthritis, it's not fun!).

The box spring and mattress that go with the Hemnes bed is sold by IKEA for hundreds of dollars which I can't afford.

What I need to do is...continue to shop for a hollow box spring. (I know they exist because I used to have one--it was stolen!). Or...I could just buy some flat (normal) bed slats and replace the curved slats with flat ones that won't make the current box spring roll back and forth....BUT...I phoned IKEA about this, and the man on the phone said it was not advisable to do this. He implied that the bed would fall apart or something like that.

Anyone know anything about this? Any suggestions?