33db reduction is about the best you're going to achieve with protection that goes into your ear, be it foam or the molded, custom plugs.
The advantage of custom plugs is more comfort than it is really effectiveness. FOr some people, things like hearos and other brands of foam plugs are uncomfortable over the span of several hours, and there is only so much noise reduction you can achieve with plugs since not all sound discerned by your eardrum enters thru your ear canal, surrounding tissue and bone transmit sound energy as well. Particularly lower frequencies, which fortunately are a less painful, though can be as damaging, as high under certain circumstances.
To achieve higher than 32-33db reduction, you have to go to things like noise cancelling or over the ear protection like shooting ear muffs, which is not an option for riding.
My advice is to try several brands of foam earplugs rated at 33 db (like Hearos blue) and to keep in mind that db reduction values are a bit non-intuitive. I've used different foam plugs next to the same loud machinery noise and the lower rated one's were noticeably "quieter" than the high rated one's. Often the material used and how completely it seals the canal (as well as other odd attentuation anomalies) can make lower rated plugs work better for you specifically than higher rated ones, softer, more porous material deadens the sound less effectively than more dense harder foam, but the softer foam fills in all the nooks and crannies in your ear to block sound leakage better than the stiffer, more dense material, and the softer material is more comfortable. Just one example of how a "better" rated plug might work less effectively for you, and be less comfortable too...
I've used those custom molded earplugs and for me specifically, they tend to stretch my ear canal painfully when used over a period of time (inserting and removing) Ironically, the 3 cent foam plugs you buy by the hundreds work better and are far more comfortable for me than $100 custom plugs. But for vanity, the custom plugs are the bomb because you take your helmet off and yank on the flourescent cord attached to the custom plugs and you look like an F1 driver as you enter Starbucks.
