The sand flea is based on the idea of running your piece of wood over the exposed drum which sticks above the table much like a jointer. With a drum sander on the other hand, the work is run between a a table and the drum like a planer.
I've seen the sand flea and tied it once, though not as in building a project. I'm not convinced in the effectiveness as it's marketed. I would assume it would have to be constructed much like a jointer with a out feed table slightly higher than the in feed to compensate for the material removed if you want your stock to stay the same width. Of course it would function with a same hight table, but repeated passes would leave you with a taper. Of course if you're building it yourself you could easily make the needed modifications.
My sander is a drum sander thus the stock is sent under the drum and rides on the table. Thickness is adjusted with a screw system to raise or lower the table which is hinged at the rear. Mine is 30" wide and I was having a problem with flex on the far corners due to the width. I came up with a duel screw system that raises both sides equally simultaneously. It's a bit crude right now and I plan on cleaning up the design, but it works great and is dead nuts on! A narrower sander wouldn't have the flex problem like a wider one and a single screw design would work well.
Goggle drum sanders and you'll find a lot of different designs, some fairly crude, others down right ingenious. I took what I thought was the best of all the designs and came up with my design.
About the Velcro sandpaper... spent a lot of $$$ on it and hated it. The problem is the Velcro flexes and leaves all the sides slightly rounded over as it flexes to conform to the work piece. The Velcro also wore out quickly also and the paper is expensive. Total waste of money IMO I now buy my paper in 25yd J weight rolls for a LOT less. I looked at and tried a number of attachment methods, the simplest and most effective I found was simple heavy duty thumb tacks! Works great, I've never had a failure.
Mike