My thought exactly. I created one from corrugated carton and the sound is sweetly muffled and, in a sense, beautiful!!! I've been thinking for awhile about making one out of Legos but, apart from the fact that I don't have Legos, the fear of din prevented me from doing so.
You could laminate some sort of foam, plastic, rubber, or other thin, shock-absorbing material internally. It should absorb enough shock to quiet things a bit without making it not work properly, no?
The roof tile pieces work perfectly to create the ramps on the inside. I used a long plank for the bottom ramp to give the dice a little extra momentum out of the base.
I don't really have plans for it, I just grabbed a huge box of legos from when I was a kid and went to town.. I'm not sure if they even make the castle wall pieces anymore. I can tell you that it works great and the noise isn't actually that bad... it's pretty satisfying.
You've since inspired me to make 2 of my own. I don't have as many castle wall pieces as yourself, but mines a little wider. It also has a trap door/lever mechanism for rolling the dice. Very satisfying.
It's more of a Dice Castle.
I made a second one in all white with a few cannons bristling out. It's my Ivory Fortress.
My main problem was that we play alot of twilight Imperium so the 10 sided dice don't show perfectly even on the lego's naturally bumpy surface. It was tough to round up enough colour coordinated smooth pieces for the collection base.