By SLP, what type of bed do you have? Does it have wooden slats at the bottom etc? We use Japanese silk rope for arms during most of our sessions and use a spreader bar attached to a rope at the bottom of the bed for leg restraints. Probably best for you to look at something similar to this https://www.lovehoney.co.uk/product.cfm?p=31705
Rather than getting one under bed restraint, we found that getting the ones which have four separate ties and cuffs work better for most beds, unless you have a divan! Haha. We even bought some cheap picture hooks once and screwed them into the bed to give us the ability to tie anything to them.
Or ask on the pander to me thread for a larger set? Maybe a brand like sportsheets will do a larger size as I know they used to have some that fit a queen bed so maybe they have something for king beds too. Couldn’t say for sure though.
I only have a regular double mattress at the moment but there really is a lot of slack. I don't want to say for definite that they'd accommodate a superking, but I wouldn't be surprised.
Alternatively you could get two sets while they're in the Bondage sale and lay them sideways. 🙂 By my reckoning you could get two sets for just over £40 for the both, or about £23 if you have an Unlimited £20 off voucher. Bargain.
I recommend that you ignore the illustration on the product page that suggests fitting them down the side of the bed. Also, fit them with the cuff straps connested under the mattress.