All of the models on this page are using at least one 5 gallon paint bucket (my "double bucket" version uses a second bucket to hold the ballast). The bucket weighs 4 lbs. The bucket is 10.5 inches at the bottom, 11 inches diameter at the top, and the lid is 12 inches across. The bucket is 14 inches long. I use 20 lbs of steel weights for ballast.
Full scale size
Scaling 11 inch diameter by 14 inches at 1:25 scale factor gets to 23 feet diameter and 29 feet high. If the inside was 22 feet across then each floor would be 11*22*pi=380 sq-feet. If there are 3 floors inside that is 1140 sq-feet. This is really huge compared to the average live-aboard boat and even more space than many multi-million dollar yachts.
Wayne has proposed a 50 foot high cylinder. For that this model would need to scale by 43. Scaling by 43 means the double bucket model was doing alright in some big waves. Also, the video should be slowed down by 6.5 and it is only slowed down by 4, so really the motion would not even be as fast as we are seeing in the video. So probably the motion of a 50 foot high cylinder with some kind of truss or hanging ballast with some heave resistance would be OK.
Videos of experiments
Single bucket version with video shot from tripod on nearby pier.
Same, as seen from the camera I am holding in the water.
This is the "double bucket" version which uses a second bucket underwater to hold the ballast. The lower bucket has much more drag than the weights alone, so does not move through the water so much (up/down/sideways). It also has more inertia since the mass of the water inside must be moved when the bucket moves. There are 4 ropes connecting the two buckets. Idea is the weights and ropes keep the bottom bucket upright and the 4 ropes keep the top bucket at the same orientation as the bottom bucket. Also, if we had slip-forms to make one concrete cylinder for the top, making a second for the bottom might not be too costly. This seems to work better than simple 20 lbs ballast.
Double bucket with weight on top (I think 5 lbs but maybe 2.5 lbs).
This next video shows the underwater view of a hanging ballast experiment. This is with 2 steel weights of 10 lbs each.
This video shows some dry land testing. First I just swing it like it swung in the water. Next we balance it on a bar and see how much weight on a side it takes to tip it. Then we try 4 separate 5 lbs weights (so same total of 20 lbs). This is not stable at all and can not even balance the bucket.
There is an Deflatable Boat Fender that is 1 foot diameter and 3 feet long. This might be a good model and it is only $67. There is also 1 foot diameter pipe, if I could get just 2 feet of it that would be interesting.