If you use 4-6 lbs or even 6-8 lbs 1/4" sides it should be fine. If one looks closely, the cross section of the model isn't really square but the sides are subtlely shaped to curve inward top and bottom to meet the curve of the top and botttom blocks, IIRC. This removes a lot of material.

Blocks as well. It might take a whole lot of time to acquire the lightest stock, but that's part of the fun of building a nice acro ship. When I shape blocks I tack glue them to the structure, finish shape them and then remove them from their position and hollow them out with a Moto-Tool so that I can see light through them. Never much more than 1/16th to 1/8th inch is left.

If you have the ability to form the top and bottom blocks from sheet, that'll be great. Whatever construction technique one chooses, the weight of the selected stock dictates the weight of the model. Every sheet of 1/16th, every block or scrap gussett should be cut from the lightest stock you can find. I wouldn't use anything but 4-6 lbs or 6-8 lbs density wood for your model. Built as per the plan, and with a lightweight finish, it should be a nice weight and fly well.

Chris...