[drache]

nice rack , Elliot
it'll look even better when it's all filled up
just estimating from what you've got on there, I'm guessing that it would hold enclosures up to about 36"x18" - not bad at all

I've used bricks too
now I use the free weights with the hexagonal heads, weight wraps and when we moved here I found a bunch of weight disks in the basement - the kind people put on bars - they're great as screen top ballasts because they're flat
I do also use clips
in a pinch, electrical tape will do - it can be reused for a long time, if treated gently

and I use pieces of different thicknesses airline tubing to stuff gaps and wedge things tighter

I also use cut up pieces of cork coasters to level tanks and furniture since I always live in places that are old and warped