-- "Light clothing" does not offer much information when projectile testing and is generally a waste of time.
-- "4 layer denim" (4LD) is a much better test of handgun bullet ability to robustly expand than "heavy clothing".
-- When using supersonic rifle projectiles, any cloth barriers are fairly irrelevant and don't change test results.
-- Get some old soft body armor panels and use them to wrap the gel block to contain any projectiles that exit the block.
-- If during a test shot an accurate chrono reading is lost, fire 5-10 shots from the same box of ammo to get an average velocity measurement so you have an informed estimate of potential velocity; note that particularly with rifle loads, sometimes it helps to move the chrono away from the muzzle a bit...