What sort of bed do you have? If you've got a wooden one, you could screw in a couple of large hooks/loop things underneath and attach a leather/fabric strap.
I've also seen sheets with straps built in which wouldn't be so hard to replicate. Actually, you might even be able to do that by putting the strap underneath the mattress and just have a bit each side to attach to the handcuff or wrist cuffs etc.
I can't find the sheets (can't remember where I saw them and don't seem to be able to find the right search term in google!) but I have found bed restraints at half the price than the ones in your links if that's any good?