I was recently looking for the same thing, and nothing met my satisfaction (can't create, can't afford). So I came up with the idea of buying a large heavy bathmat. My son is still small enough to fit under, and I throw it over his sheet. The kind I have doesn't have the rubber, because that won't work with this summer heat, but it's still heavy.
It's been only a month but he's had a huge reduction in nightmares and "sleeptalking", and he asks for it now.
Thought I'd pass that idea on in case it could help someone.
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Be not angry that you cannot make others as you wish them to be, since you cannot make yourself as you wish to be.
~Thomas à Kempis
"Be plain, good son, and homely in thy drift;
Riddling confession finds but riddling shrift"
~Shakespeare