okay- don't get me wrong, i'm a sucker for a beautiful pair of cuffs or a leather paddle, but i absolutely love Do-it-yourself BDSM. it's like a hobby.
if you can get something around the legs of the bed, that will be much sturdier than just under the mattress. if you don't want to buy a bunch of chain and rope, i recommend getting some (4) great dane-sized choke collars (or smaller depending on how long you need them) and some zip ties. zip ties are absurdly cheap and strong. only bad part is, once it's on, you have to cut it off (it's not going anywhere otherwise which is a plus) and they can't go on a person- but beds are a go.
the choke chains are pretty cheap and come in easy lengths with loops at the ends to attach cuffs. they're also not so bulky that anyone who stops in your room will see your torture chamber.
so run the zip tie through one end loop of the choke chain and then run the zip tie around the leg of the bed (if the leg is wide with your bed, they make huge zip ties) then attach your cuffs to the other end of the chain. and if you want more versatility than that, if you get a few hook clips from your hardware store and zip tie that on instead (they're like 20 cents each) you can then attach different length chains, rope, anything to each leg and unclip it for storage and to keep your room PG looking.
hope that made sense and helped! DIY BDSM is way too fun.