help on basic frame stability

hey guys

I'm building a prop for church ( the Roman Centurion helmet and sword are "artistic license" ;))
pastor wants to take a white sheet and suspend it for painting/projecting
it roughly 6' x 6'. Of course the top 2/3's will get used the most, as you don't want to bend down to reach the bottom.

So I've sketched up this basic rolling frame. Built of pine 2 x 4's - nothing fancy as it will get painted and does not need to support a lot of weight.
The uprights are attached to the base with a through-tenon
Top brace attached to uprights with mortise/tenon.
The wheels will be double-locking. The bottom braces they are attached to are 15" wide.

two questions:
1. does this seem stable enough, or top-heavy? I can't brace anything in the area of the sheet as light must be projected through
2. I need some way to to tighten the sheet. I put a mockup of a handle on the bottom edge. I'm envisioning some type of ratchet mechanism, but I can't figure out how the dowel rod will be locked after the sheet is tightened. I found Rocker item#90910 but I don't see a way to lock the handle with respect to the wood upright. any ideas?

thanks

image attached

how do you get a sketchup model under 98 KB?