thank you all for your comments.
with the selection of silicone, I have chosen an aquarium type, so I am sure that I am good on that front.
DavidY: I have considered the 'carpets' and drawer liners, but I am not sure how easy it is to clean them? Afterall, the surface does come in contact with water and all, and I don't know how it will work through time.
As to the design, I think I may build another one in the future and just have one level and a longer ramp, as my tank is too narrow to actually provide the upper level a large enough surface area for basking.
Whatever you you use, I'd use aquarium silicone to attach it and let it dry for 48 hours.