OP didn't say how old her child was. As @Mambo said...might be useful info for others with younger bubs😊
Our piece of foam was a perfect fit, it was only a thinish piece, and i didn't see any risk. He did just fine with it. The "matress" that came with the portacot was hard and barely offered any padding. He used it from about 3 months when he outgrew his bassinet. He was still in our room though right next to my bed so i was always keeping an eye on him (cot wouldn't fit in our room so we used the portacot temporarily)
If you take the existing cot mattress to Clark Rubber they cut the foam to be the exact same size. There's no gaps between the mattress and side of the cot. Use a mattress protector and bottom sheet and I felt it was firm and safe.
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