I've been using one of
these recently, fairly cheap and not the best quality I've ever seen but does seem to do the job, I've had many leaks some quite big and not noticed any wetness on the mattress afterward, these go on like a fitted sheet but unlike some others the waterproofing goes around the sides too not just the top.
Pretty much silent and easy to put on and off under other sheets, one bigger drawback to this type is that the bed and top sheets definitely feel wet and it can spread a fair bit but it does work to protect the mattress.
fitted waterproof protectors like this can work well to save a mattress and can come as either "fitted Sheet" style or fully enclosed zip around. they can use different material for waterproofing like PVC, Vinyl or PUL and if you really go digging you might be ably to find some older style Rubber ones. they have different levels of effectiveness and noise level with PUL being probably the best all round but more costly.
A bonus to these is that they look just like any other normal fitted sheet/cover when drying on the line, you would never tell its waterproof just by looking at it.
I also have a couple of pads like
these which are also commonly known in the care industry (at least in the UK) as "Kylie Pads". they have a thick absorbent core (these ones say they hold 3000ml for a single size) often with a feel dry top layer (like a cloth diaper) and a waterproof bottom layer, two sides have "wings" which tuck under the sides of a mattress to hold them in place making them even easier to fit and remove. They can be very effective if you get the right length and positioning and unlike the other type if you can get the slightly more expensive ones with a feel dry top they don't feel very wet.
The downside of these is that if you get the position wrong, wriggle around a lot or have a really heavy wetting they can leak off the edges and onto the mattress so they are best used with a cheaper fitted sheet type to protect the mattress then a kylie pad on top to soak any wee up and make cleaning/changing the bedding easier (if it hasn't leaked onto the protector just change the pad and sheets). Another downside is that they are pretty obvious what the are when drying on a line.
***Edit to add.
The kylie pads are very much like the disposable protectors you can buy just washable but as an added bonus depending on the size you need may also be usable for chairs, make great changing mats and can easily be taken to other places you may need them (camping, hotels etc.)
